A general method for calculating spacecraft collision probability is developed. In this method the input required to perform a calculation includes the respective state vectors, position error covariance matrices and physical sizes of objects involved. The method is valid for the general case because it only relies on the general form of error covariance matrices. The relative velocity is assumed to be vector having constant direction, and then the collision probability problem can be reduced to two dimensions in encounter plane normal to the relative velocity vector by eliminating the dimension parallel to the relative velocity vector. Three kinds of method resolving the two dimensions problem are developed and compared. Test case results indicate the method is valid and applicable.