The invention discloses a software polarity determining DC electric field measuring system based on speed closed-loop control. The system comprises a signal processing module, a signal bias module, avoltage comparison module, a motor speed control module, a speed measurement module, a speed regulation module and a controller, the electric field signal processing module collects and amplifies measured electric field signals in an anti-phase way, electric field square wave signals are output from one path of electric field signals after voltage comparison and input to an I/O port of the controller, and the other paths of signals are input to an A/D channel of the controller after signal bias; the motor speed control module outputs a grating signal, and a speed signal is obtained via the speed measurement module, the speed signal is transferred to an external interruption port of the controller, and when the speed signal causes external interruption, the controller outputs a PWM signal via the speed regulation module so that the rotating speed change of a motor is consistent with a preset value, and closed-loop control is realized; and in the period that the grating signal causes external interruption, the controller determines the polarity of the electric field according to the level state of the electric field square wave signals and the frequency in which the electric field signals pass an amplifier.