A linearity measurement systems of accurate photodetector is described. The principle of linearity measurement methods is dicussed, and the beam-addition method is used in this measurement system. Structure of measurement system is designed, and the linearity measurement of Si trap photodetector and InGaAs trap photodetector is done with 944 nm laser. Experimental results show in the range of laser power (0.1～200 μW), the measured nonlinearity factor of Si trap photodetector is less than 0.009%, and its combined uncertainty is less than 3.18%, the measured nonlinearity factor of InGaAs trap photodetector is less than 0.6%, and its combined uncertainty of measurement is less than 6.87%. Experimental results prove that the system can be used in accurate photodetector’s linearity measurement.