In order to make an understandable illustration for undergraduate students，an improved method of explaining the diamagnetism of materials is suggested.First， the mechanism of the diamagnetism is interpreted by theoretical derivations when the orbital magnetic moment of electron is parallel to the external magnetic field.And then， the properties of the motion of electron is analyzed by employing numerical calculations in the case of orbital magnetic moment of electron not parallel to the external magnetic field.The numerical results are illustrated and displayed as figures，which reveal the mechanism of the diamagnetism in this case.