During the imaging process of space camera, small fixed gains are often used to avoid image saturation. As a result, obtained images have problems such as dark as a whole, short of digital number levels. An automatic on-orbit adjusting gains method of space camera based on lighting conditions is put forward. In this method sun zenith angle at substellar point is calculated in real time according to current position of spacecraft and time. Function relationship between sun zenith angle and radiance at entrance pupil of space camera corresponding to maximum reflection index of targets is established by non-linear curve fitting. In this way real-time calculation and adjustment of gains are realized. Results of contrastive experiment with results of simulation by satellite tool kit (STK) and computation by MODTRAN indicate that radiance fitting errors are no more than 0.3 W/(m2·sr) as sun zenith angle belongs to [20°,70°].Relative errors are no more than 2.2%. Difference error between calculated sun zenith angles and those simulated is about 0.39°(3σ). Results of on-orbit imaging experiments indicate that digital number levels are improved from 98 to 183 and entropies are improved about 19.2% after adjusting of gains as sun zenith angle is 62.5°. Targets will be easier to be recognized as more digital number levels can be obtained.