Soft starters are effective devices used to provide overload protection for motors from large mechanical shocks during the start-up period. However, existing soft starters require additional power supplies, sensors, and complex control elements that pose serious challenges to the integration, versatility, and operability of mechanical transmission system. Herein, we propose a newly soft starter based on the triboelectric nanogenerator (TENG) and electrorheological fluid (ERF) to realize a self-powered mechanical transmission system. Both ERF’s rheological characteristic and the baffle structure play a role in the torque of device. Driven by TENG, the soft starter with optimized baffle achieves a 715% growth in transmission torque compared to that of the device without baffle. And a smooth start is obtained with transmission speeds ranging from 0% to 100%. In application demonstration, this triboelectric soft starter (TSS) has the capacity to gain a smooth operation of the high-speed motor. In contrast, the direct start generates an overshoot, leading to a break in the conveyor belt. The TSS designed in this work with the advantages of self-powered, highly integrated, easy to operate, and low cost, provides a prospective strategy for broadening the application of TENG in mechanical transmission systems.