The speed of rotating mirror must be measured in shooting period precisely in order to obtain the shooting frequency of framing cameras or scanning speed of steak cameras. A high precision digital measurement method based on digital signal processing (DSP) and field programmable gate array (FPGA) is put forward. DSP and FPGA are communicated by high-speed serial peripheral interface (SPI). Time gate for FPGA is preset by DSP according to the change of rotating mirror′s speed. The measured data is transmitted to DSP for disposing after FPGA′s counting and buffering and to computer for showing in real time. It prevents the overflow of the data and improves the range of rotating speed by adopting 32 bit logic unit for FPGA. High-speed DSP improves the process speed of the data and ensures the real time. The design is feasible by the simulated result of the system and the engineering practice. The precision of speed measurement is up to 0.0001%, and the range is from 3 to 3×108 r/min.