The principle of measuring nonlinearity of ultraviolet photodetector and power meter at 266 nm was presented in this paper. A new technique is proposed for measuring the nonlinearity of ultraviolet photodetector using the power superposition method. The facility of accurate measuring ultraviolet photodetector nonlinearity has been developed. The power nonlinearity dynamic range is widened by using a polarizing beam splitter cubes for beam splitting and combining. The effect of background and scattering signals on the measurement results is reduced by the light shield. The measurement uncertainties are reduced by automatic test programme and optimized model. Results show that in the power range of 1 nW～100 μW, the repeatability of the results is better than 0.3％ and the typical expanded measurement uncertainty of nonlinearity is less than 0.8% (k=2).