On the basis of the analysis of the characteristic of the Photo-reconnaissance Satellite, a model was built which describes the interactions among the probability of target detection versus the satellite's inclination, the detection width, the max access gap and the target characteristics. The model formulates the relationships between the technical index and the tactical index, and can be used in the requirement analysis, system design and effectiveness evaluation of Photo-reconnaissance Satellite. The simulation results which takes the large-scale ships as detection target are given to illustrate the applicability of this model.