The detecting performance of an airborne daytime star sensor for navigation is analyzed and a daytime star sensor prototype is researched. Mathematical models of Signal to Noise-Ratio(SNR) and contrast of a photoelectric imaging system in daytime under noises are established, and calculated results by different focal lengths and different apertures are compared to conclude the influence of system parameters on the detection results and the requirements of optical system.The conclusion shows that the proper relative aperture of optical system is 1/15. A daytime star sensor prototype for tracking stars of magnitude 3 is designed. Tracking images of the polestar from sunrise to noon are acquired by a ground experiment using a F/15 objective, and brightnesses and contrasts of star images at different time are compared. It shows that the contrast of star image has descended from 1.55 at 5:00 a.m. to 1.2 at 12:00 a.m. The minimal degree of modulation is more than 0.03 by calculating with the prototype′s parameters, which shows that the optical system with Φ70 mm and F/15 can satisfy the detecting requirements. The theoretic analysis and design is proved to be available, and the research can be a reference for developing of miniaturized and precise airborne daytime navigation star sensors.