In order to explore the flow characteristics in horizontal mutation pipe, the SIMPLEC algorithm and the computational fluid dynamics software Fluent are used to make a numerical simulation on the mutation pipe. The effects of different entrance velocity, pipe diameter ratio and inclination angle on the reattachment length and local resistance coefficient are analyzed. The results indicate that the reattachment length is related to expansion ratio and inclination angle; the local resistance coefficient increases with the increment of expansion ratio whereas decreases with the decrement of the inclination angle; The results reflect the essential characteristics of mutation pipe flow and provide a reference for solving the common problems in industrial production. Keywords ：mutation pipe ；expansion ratio ；mutation pipe inclination angle ；reattachment length ；local resistance coefficient