The titanium alloy strut is a key load-bearing part of aircraft landing gear, and a forging process is adopted to manufacture the strut. The friction condition has important influences on material flow and cavity filling in the forging process. Using the previous optimized shape and initial position of preform, the influence of the friction condition (friction factor m=0.1 – 0.3) on material flow and cavity filling were studied by numerical method with shear friction model. A new filling index reflecting the material flowing into left and right flashes and zoom the results caused by friction was defined. The results indicated that the preform/workpiece would move rigidly to the right direction, and the moving displacement would decrease with the increase of friction factor m; when friction factor m<0.18, the underfilling defect will occur for left side of strut forging but overflow occurs for right forging die cavity; by using the filling index combining analyses of material flow and filling situation, the reasonable friction factor interval being from 0.21 to 0.24 can be determined; the cavity filling behaviour is robustness and the friction fluctuation has less influence in above reasonable friction factor interval.