if the centripetal force acting on it then it must fall on to the earth ,why it is not falling?

yes it is true that a body such as the moon revolving around the earth, moves in its orbit with velocity “v”, then the force acting on the body is in the direction tangent to the orbit in the direction of velocity vector. this force is equivalent to the centripetal force acting on the body....hence the equivalent force on the body becomes zero.

hope you are clear now....