Based on laser Doppler effect and principle of rotational dynamics, a novel approach to dynamic torque measurement is proposed. Mathematical model of measurement method is deduced. Four highly coherent laser beams irradiate four points of two sections of rotating shaft by designed optical measurement system. Backscattered lights with Doppler frequency shift are mixed onto the surface of photo-detectors, heterodyning takes place, and the output current is modulated. The photocurrent frequency difference is proportional to rotational velocity. Speed fluctuation values of rotating shaft segment and relative speed fluctuation values of between sections are obtained directly by measuring instantaneous rotational velocity of double sections in two successive times, and then dynamic torque values are obtained. Experimental results indicate that the identification values of dynamic torque are improved 5%～10%, which reflects the dynamic characteristic of torque fluctuation. Compared to conventional measurement approaches, this approach can promote the dynamic measurement range, and is more real-time.