Contrast of star to sky background is an important parameter of star sensor. It determines stellar magnitude detecting limit of the star sensor. Method for calculating the contrast of star to sky background is described. For a star sensor with 3°×3° field of view, 0°and 40°observing angle to detect 6.0 magnitude stars of K and M types, the contrast of star to sky background is calculated as a function of observing height,solar zenith, wave band, and stellar spectral type. Results show that the contrast increases exponentially with increasing of observing height, and the value at 40 km altitude is about one hundred times of that at 10 km altitude. The contrast also increases nearly linearly with increasing of solar zenith, and the value at solar zenith of 80°is about six times of that at solar zenith of 20°for longer wavelength. The contrast in 0.4～1.0 μm spectral range by using B+W090 filter is close to that of 0.6～0.7 μm waveband, and the contrast of the whole waveband almost doesn't vary with stellar spectral types. The results can be used for estimating stellar magnitude detecting limit of the star sensor in the atmosphere during daytime.