With the high-speed development of science and technology, the demand for information is growing rapidly. Space laser communication is used more and more widely due to its advantages, such as faster rate of communication, smaller beam divergence angle of communication and better security and confidentiality. In the space laser communication system, the sky background radiation is the main background noise. Spot extraction has become a major impact on tracking accuracy in strong sky background conditions. Several methods for suppressing sky background were described, such as using atomic filters, reducing the caliber receiver, et al. Under the experimental conditions, coarse tracking spot is processed by using Otsu thresholding and centroiding algorithm. The effects of sky background light on coarse tracking spot are analyzed. The experimental results show that under the influence of the sky background, the Otsu thresholding method can accurately extract imaging spot, it also can effectively improve the coarse tracking accuracy.