The factors affecting collision probabilities between formation-flying satellites are discussed and generalized, for the purpose of increasing the accuracy of the collision detection and forecast. A difference equation of Hill's equation is derived for the Kalman filter, which is used to estimate the target satellite's status. Based on these equations, such four factors as model errors, measurement errors, malfunctions and accidental events are analyzed in detail. Several simulation scenarios are also executed for the factors. The simulation results show that the controller always-functioning malfunction is the worse collision factor, which needs further researches for a specific collision avoiding scheme.