The relationship between the forming load and the forming process parameters greatly affects the stability of the forming process and the performance of composite stringer preforms. By applying the domestic composite material L-shaped long automatic forming machine to build a mechanical forming load test system, the variation law of forming load was tested and analyzed through the study of molding temperature, molding rate, forming pitch and layup structure, which achieved an empirical formula between process parameters and forming load. The results show that, as the molding temperature gradually increases, the molding rate gradually decreases, on the other hand, the molding pitch gradually increases, which causes the decrease of molding load. When the lamination method is different, the molding load changes as follows: [0°/45°/90°/–45°]_ns>[0°/90°]_ns>[±45°]_ns. Finally, an empirical formula for the relationship between the forming load and the process parameters is obtained, and the molding temperature, the forming rate, and the forming pitch can be practically guided within a certain range.