A three dimension numerical simulation software named NUMECA(numerical mechanics application) was used to calculate the tip clearance flow of fan/compressor. The flow field performance under design speed condition was obtained. The results show that when back pressure of by-pass duct increases, the intensive blockage area moves upstream and towards the fan pressure surface, and twist of shock become more obvious. The efficiency and the mass flow of the by-pass duct decrease if the back pressure of the by-pass duct increases unceasingly. Atcore duct, the area influenced by tip clearance leakage flow spreads to upstream with the increase of back pressure. In the same way, the efficiency and the mass flow of the core duct decrease if the back pressure of core duct is too great. The comparison between the results and the experiments shows that the numerical simulation results of three dimension viscous flow of fan compressor with tip clearance could actually reflect the influences of leakage flows on the flow structure and parameters.