In order to obtain an accurate centroiding algorithm, it is inappropriate to directly regard star image energy distribution as the Gaussian distribution because of the upgrading demand of ultra- high precision star tracker. It is urgent to study the actual star image energy distribution. This paper studies the theory of mathematical statistics about discrete data model deviating from the normal distribution. The skew Gaussian distribution is proposed to describe star image energy distribution of high precision star tracker. Both simulation and measuring experiments prove that the actual distribution of the star image get closer to the skew Gaussian distribution. The results can be used to guide the research of the precise correction algorithm of the star tracker which can correct the coordinates of the centroid.