The explicit expression of the collision probability is deduced under the assumption that the orbit is a circle. The collision probability is expressed as an explicit function of the encounter geometry (crossing altitude difference and time difference of the line of intersection of the two orbital planes, orbital planes included angle, etc) and position error variance in RSW coordinates. With the help of the explicit expression, the influencing factors of the collision probability were analyzed. Effects of factors such as altitude difference, time difference, orbital planes included angle, position predicted error variances and the equivalent radius of two objects were discussed. Some significant conclusions were obtained.