Abstract: With the expansion of the human＇s activity in exploring, utilizing the outer space, protecting space environment have become the most significant task in space navigation. Since the neglect of the space debris environment in the past century, the total number of orbital space debris increases rapidly, which leads to an unacceptable large risk of collision to spacecraft. Space Debris Collision Avoidance （SDCA） is the only strategy to reduce the risk of collision, when the dimension of space debris is lager than 10 cm. The decision whether the avoiding maneuver is necessary depends on the analysis of conjunction, the rigorous calculation and assessment of collision risk between spacecraft and debris and compare to the collision criterion. This paper initially introduces the flow of the Space Debris Collision Avoidance System （SDCAS）, with comparing of the two kinds of warning criterion, the main topic on method of collision probability is evoked. Collision probability is a new type of warning criterion, which not only takes the miss distance into consideration, but regards all the geometrical parameters during the approach as the factor to the collision risk, combining with all influence of these parameters. Eventually, with the detailed analysis, the effect of the covariance or uncertainty to the collision probability has been drawn to the surface, which plays an indispensable role in measuring the risk of collision. The attitude of the error ellipse which is constructed by the covariance has also affect the result of collision probability. Based on the analysis of covariance, the method of collision probability posts many advantages to the Box method. Especially, the rate of false warning has been decreased dramatically.