Estimating formula of the smallest power of illuminating laser of tracking, acquisition and pointing system in space is founded, and the smallest value that the illuminating laser sends when the receiving system can detect the return signal is obtained. The radiant intensity and the radiant quantity reflected by the diffuse reflection target of secondary planet are researched in theory and simulation. It is concluded that the distributing area of radiant emittance and the radiant intensity are the same as the figure of surface of projection in section of laser beam. The factor which affects radiant quantity of secondary planet is the effective area of projection, which includes the reflecting area of secondary planet and the angle between surface of projection and reflecting surface. The factors which affect the radiant intensity are the area of projection in section of laser beam and the angle between the direction of laser beam and normal of reflecting surface. The radiant intensity and quantity of secondary planet affect laser power of receiving system directly.