Abstract: In order to analyze the distortion mechanism of the magnetic field inside a long solenoid driven by square wave, the magnetic field distribution inside and outside the solenoid was studied using Maxwells equations. Internal and external electric field, magnetic field model of long solenoid driven by sine wave was established respectively by using Maxwell equations. Magnetic field distribution of long solenoid driven by sine wave was obtained, selecting appropriate boundary conditions of ampere circuit law and law of electromagnetic induction. Then square wave was transformed into the form of multiple sine signals by Fourier transform, and an expression for the magnetic field of solenoid driven by square wave current was obtained. The driving signal square waves frequency effect on magnetic field was emphatically analyzed through the simulation test at last. The conclusions demonstrate that there is distortion in the magnetic field of a solenoid driven by a square wave current. The lower the frequency is and the closer the distance from the axis is, the more square wave features the corresponding magnetic field can acquire.