An experimental study on the classical experiment that verifies the momentum-energy relation in the special relativity theory is carried out and it is found that when magnetic intensity varies the experiment results do not confirm to the prediction of the special relativity. By analyzing the results, the paper points out that the postulate of special relativity on the invariation of the force acted on a moving electron is wrong, and further proposes that the Lorentz force (theoretically) acted on the moving electron should decrease as the velocity of electron increases.