Abstract: Daytime space target laser ranging data can help improve the space target orbit determination precision, has important application value in terms of space research. During the daytime, the sky background is light, laser ranging echo signal is very weak, generally to identify the weak space target laser echo signal from the strong background light is very difficult. In allusion to weak signal detecting technical problem of laser ranging for space target in the daytime, influence of sky background noise in the daytime was estimated, ranging false alarm probabilities at various detection threshold was calculated, relation of detection threshold and laser ranging gate was analyzed, detection threshold requirement of laser ranging for space target in the daytime was presented, a weak signal detecting method of laser ranging for space target in the daytime based on multi-photon detector was put forward, and its feasibility was validated by experiment. This result can be used for designing daytime laser ranging system and researching new type laser ranging system.