Abstract: The ovality of pipe fittings has a huge impact on welding quality and efficiency, and also affects the residual stress distribution of the pipe fittings, thus affecting the service life. Therefore, it is necessary to design an integrated in-pipe rounding mechanism that is compact and suitable for a certain size range. Based on this, this paper proposes an in-pipe hydraulic rounding mechanism. In order to improve the control accuracy of the in-pipe hydraulic rounding system, the genetic algorithm is used to optimize the fuzzy-PID control method. Because simultaneous movement of multiple cylinders is required in the rounding process, the error compensation synchronous control method is adopted. The simulation results show that, compared with the traditional PID and ordinary fuzzy-PID control methods, the optimized fuzzy-PID controller has a faster response speed, smaller overshoot, more accurate and rapid control of the position of the roller, and the error is controlled in the range from 0.1% to 0.2%. Inside, the multi-cylinder synchronous motion has a fast response speed and precise control, which can realize the precise rounding of the pipeline. This fuzzy-PID hydraulic rounding mechanism improves the rounding quality and provides a new idea for the design of the in-pipe rounding mechanism.