what should i do for this problem?

How about switch you cat to indoor only. Letting your cat outside is very dangerous for her. Many things can kill her and she can get lots of diseases from poisonous plants and chemicals in peoples trash cans.

just put your can on 2nd floor while you try to figure out what you can do to keep the stranger cat out from your home, I think a dog is an answer, the question is do you have second floor??

Now ... these really is a problem. I just can't find an answer. If you close your flap ... your cat can't go outside and come back inside. And if you leave it open the other cat comes and scares your cat. LOL. The other cat ... that scares your cat ... has an owner ??? If yes you should talk with him to rezolv this problem. If it is a stray cat (with no owner) you can move it to another neighberhood. I can't figure out another solution. Or... you can adopte that cat :D. And they will get along... :D

hey just bring a dog to ur home n let that be close to ur cat.so that no other cats can get in.