Influence of weld lines is important to the mechanical properties of the modified polypropylene products. Orthogonal experiments are adopted to confirm injection process parameters for such mechanical properties as impact resistance and tensibility of the modified polypropylene injection-molded parts in weld lines. The factors which affect the mechanical properties at welding joints product vary, and how to select rational parameters is a difficult problem for production. The mechanical strength can be increased through higher temperature, lower injecting pressure, higher velocity of filling, higher packing pressure and longer packing time. The specifications of cross section in weld lines of the products are analyzed to illustrate the theory of fracture. In order to increase mechanical strength of the sample, the length of hot weld lines should be increased and the length of cold welding joints should be decreased through adjusting the process parameter.