As an accurate independent navigation equipment, star sensor is widely used on different kinds of platform. The detection ability directly influences the efficiency and measurement accuracy of the star sensor. Shipboard star sensor works on the sea, which is different from the space, the detection ability is influenced not only by the parameters of the lens and the image sensor, but also by the night sky background and the atmosphere. In order to provide theoretical basis for research and application of shipboard star sensor, a calculation model of the detection ability for shipboard star sensor is deduced according to the study and analysis of star light propagation, imaging and signal extraction process, and the correctness of the model is verified by experiment. All the influencing factors are analyzed with the model. In the end, several suggestions on design and development of shipboard star sensor are proposed based on the analysis results.