Integral proportional navigation law which can be applied to strapdown laser semi-active seeker is a kind of typical guidance law. In order to analyze the precision with this kind of guidance law, the analysis based on adjoints was put forward. Firstly, the model of integral proportion navigation guidance is constructed and the implement of the integral proportional navigation law was given. Secondly, based on the analysis of characteristic of different kind of error sources, the influence of these error sources with adjoints was got. The result shows that precision has a close correlation with navigation constant N and dynamic behavior of autopilot. These conclusions are significant for the application of the strapdown laser semi-active seeker in engineering.