a electron is moving in magnetic field.why its kinetic energy is constant in magnetic field

Not in every magnetic field...here you are talking about electron entering the mag field in perpendicular direction.

In that case the force on the electron will be max and=qvB

and the electron will move in circular motion...rather uniform circular motion as there is no other force acting.

In uniform circular motion the speed is uniform . So its KE will also be same through out.

In ideal case it will dissipate due to friction.