The utility model discloses a terminal voltage measuring instrument of an AC variable frequency motor, which aims to overcome problems of larger error in converting DC bus voltage to terminal voltage, requirement for ultrahigh digital sampling frequency and high-speed digital signal processing in directly measuring the terminal voltage because of PWM square-wave characteristic, and incapability of realizing by common controllers. The measuring instrument (5) comprises a sensor, a sampling resistor, an AD sampling module and an FPGA. The input end of the sensor is connected with the output end of an inverter (2) through a wire. The output end of the sensor is connected with one end of the sampling resistor through a wire. The other end of the sampling resistor is connected with the signal input end of the AD sampling module through a wire. The signal output end of the AD sampling module is connected with the input end of the FPGA through an I/O port. The input port of the measuring instrument is connected with the output end of the inverter (2) through a sensor and a wire. The output port of the measuring instrument is connected with the input end of a motor controller (4) through a wire and the FPGA, namely the field programmable gate array.