Right now I have the basic velocity and gravity,and all is good. The problem is,when the player jumps and goes forward,the falling speed(and just the way the player falls is not that realistic).

If I increase the gravity,the player will come back to earth too fast,making it more unrealistic. Does anybody know some kind of guide for handling this type of stuff?

If you want to make it realistic, make gravity about 9.8m/s^2. If there are very large falls, you may want to introduce drag, so a falling body doesn't go beyond terminal velocity (~55m/s).

But you probably don't need realism. If your jumps don't feel right for the game you are making, do whatever it takes to make them look right. A maximum downward velocity, sideways acceleration in response to user input while in the air... It doesn't matter if they don't exist in the real world: This is your world!