The triple screw extruder is a new equipment of polymer processing. A three-dimensional modelling of a triple screw extruder with combined screws of convey and mixing elements is adopted to investigated the flow and mixing characteristics by using Polyflow software based on finite element method. The axial velocity and pressure distributions in the special central region of the triple screw extruder are specially analyzed. Some mixing evaluated parameters, such as area stretching rate, the probability of mixing index, segregation scale and residence time distribution in different boundary conditions are calculated based on particle tracing method. Moreover, the effects of screw clearance and inlet flow rate of combined screws on the mixing characteristics of polymer melt are discussed. The results show that the increase of the screw clearance can increase the residence time and improve the mixing efficiency of extruder. With the decrease of inlet flow, the distributive mixing efficiency in the triple screw extruder increase gradually.