There is a cat that sometimes comes visits me, and I give sausages to him.But he still won't let me pet him, because he goes back slowly. He comes when I call him,but still, I think he doesn't trust me. He even "asks" me to follow him, but he is scared of me.
It's probably been mistreated by someone at some point so it's not likely to trust you easily. If the cat looks homeless, giving it a nice place to sleep that's either in your house, or very near your house is a good idea. Keep feeding it and talking nicely to it. Once the cat learns that you are not a threat, it might let you pet it. It might try to get you to follow, and you probably should, just to show that you can be its new buddy.
Cats express trust and affection by softly closing and blinking their eyes at each other. Almost like if they're sleepy, or falling asleep. If you make these kinds of eyes at the cat, that's a first step. You're telling kitty that you're fond of it and you trust it. If the cat makes these kinds of eyes at you, you're making progress. If you can sit there looking at it making soft eyes, you're already buddies.
When getting ready to pet an untrusting cat, always get down on their level. Standing or even kneeling down makes you seem big and threatening, or like you're getting ready to pounce or jump up. It's best to sit on the ground in a way that it doesn't look like you're going to be easily moving any time soon; sit cross-legged. This lets the cat know that you're trying not to be big and scary, and you have no intention of doing it harm and running off, or kidnapping it. You are just two 'cool cats' hanging out eating some sausages.
So, now you're sitting there and looking at the cat like you're both getting sleepy. Slowly blinking at each other, or looking at each other with softly squinted eyes (not angry squints). Stick your hand out with the palm side facing down, sort of like if you were to add a pinch of salt to your dinner, fingers down and relaxed, maybe even rubbing together a little. The cat might be curious if you have more food that you're holding. Your hand should be at or below its eye level. You can also put some food that's close to you, but just out of your reach. That'll bring it in closer, but DO NOT ATTEMPT TO LEAN OVER AND TOUCH IT! Just let it get used to the idea of you being close. Let it associate you and food, and you and soft eyes, and you and NO quick movements.
If it's happily eating your snacks and your hand is out there, it is possible that after enough of this kind of bonding, when it's finished a couple tiny pieces of sausage, it'll sort of come over and gently fist bump your hand with its head. I still wouldn't attempt to get more pets from it than that, for now. Just let it rub its face on you and sort of 'ask' you to pet it on the head. I'd just pet the cat slowly on the head, sort of combing its fur in the right direction, maybe gentle scratches. Don't try to go under its chest, or pet its body yet. Not until you are fully able to pet it without it freaking out. DO NOT TRY TO PICK IT UP!
Slowly, the cat will get used to the pets and ask for more. If you're petting it and it sort of lies on its side like a dead cat, then you can start to pet the body and maybe the belly. I call these "dead cat snuggles". If a cat exposes its underside, that means it really trusts you because that's its most vulnerable state.
Patience. As many weeks or months of patience as are required. Move calmly, let the cat come to you as much as possible rather than chasing it, don't grab -- hold out a hand to be sniffed and rubbed against rather than trying to pet the cat, until it's very comfortable initiating contact itself. Ferals in particular will often take a long time to decide that they're comfortable with extended contact, if they ever do.
If a cat decides quickly that you're a good human, then either it has been lucky enough not to know any bad humans or you should feel very flattered. Or both.
Think about it from the cat's point of view. King Kong over there seems friendly, but how sure are you? Trust takes time.