Abstract: In this paper the variation of rigid part's pose errors and assembly forces during the compliantassembly process are analysed.The paper explains the unnormal situation of peg-in hole assembly and dis-cusses the feasible region to perform assembly. The practical way to improve the assembly effects are sug-gested.The mathematic models and constraints given in the paper not only are the theoretical basis of forcemotion planning in robotic assembly task,but also serve as the foundation of the research of all kinds of externalcompliance technology.