The dynamic equation of electron in the experiment of measuring charge-mass ratio of electron by magnetic control method is analyzed, and the theoretical calculation formula of measuring the charge-mass ratio of electron by magnetic control method is given. The electron trajectories are further simulated by using the dynamic equation. The calculation results show that under the condition of non-zero cathode radius, electron motion exhibits a kind of periodic motion such as alternating curve of many rotating heart-shaped lines. By means of simulation calculation, it is verified that the theoretical formula obtained by us is basically consistent with the experimental results of dynamic equation simulation, thus verifying that the theoretical formula given is correct and universal.