Magneto-optic modulation based on square wave signal has excellent properties, while it has the disadvantage of signal distortion. Theoretically, its mechanism is studied. Square wave signal is expanded into different frequency sine signal form by Fourier series. Magnetic fields of long solenoid driven by various frequency sinusoidal signals are calculated with Maxwell′s equations and Bessel′s equation. Various magnetic fields are added together and magnetic field of long solenoid driven by square wave is calculated in theory.