RigidBody2D Add an impulse force not working in horizontal direction, only when control player bby velocity!

I'm writing a simple 2d game and want the player to be hit back if something hit it, Just get the velocity of the hitting object and applying an opposite impulse force to the player. But that doesn't work. But what i find weird is when i choose to use AddForce to control my character, all my previous code will work perfect fine. So i guess there're some problems with Unity Engine.... And what make me annoying is it's not suitable to control player using AddForce, cause it will be accelerating so that not a willing move= =. Really annoyed by this, please help me out guys. Thanks!