Abstract: There is no good solution to the torque measurement of mechanic principal axes working under various loads at home and abroad. Through installing the circular grating and indicative grating at both ends of a principal axis, the noncontact dynamic measurement of the principal axis torque can be realized by computing and subdividing grating Moiré fringes. After the photoelectric conversion of the Moiré fringes, the signal is amplified, filtered and compared with integrated programmable analog devices. Here, a microprocessor of soft core is used to realize data acquisition, processing and controlling, and to substitute the method of FPGA+MCU. During the experiment, the circular grating with 1200 grating lines was adopted, and the precision of the twist angle (less than 0.001°) was acquired while the rotational speed of the principal axis is within the range of 0～1500r/min. The experiment shows that the adoption of Moiré fringes of the circular grating can make highprecision torque measurement of the principal axis meet the requirement, which offers a new noncontact method for the measurement of mechanic principal axes.