Abstract: Traditional researches on payload separation from spacecraft don't consider actual contacts in rail clearance, and separation velocity of payload can't be assessed accurately. According to two semicircle rails, characteristics of contacts between spatial rail and director were investigated. A method for detecting potential contact points between director and rail with relative movements was presented based on the structural features. Spatial angle between the director and the rail is calculated to detect contact exactly in the later stage of separation. The normal contact force was modeled using the continuous approach proposed by Lankarani and Nikravesh, and the tangent friction was calculated by a modified Coulomb's friction law. Validity of the methodology is proved through a free-floating flexible spacecraft model. And the results show that rail clearance causes high contact forces, and velocity vertical to the separation direction increases with clearance size. Further more, clearance aggravates the coupling between the appendages and the base, and effects spacecraft's stability.