Based on the analysis of the position control system of hydraulic screw down for a POF three-roll pipe mill, the load pressure, load flow and leakage flow are redefined, and the transfer function model of the piston rod of the hydraulic cylinder is deduced when the piston rod is extended and contracted.A theoretical basis was provided for the establishment of symmetrical valve-controlled asymmetrical hydraulic cylinder mode.The screw down system of tandem rolling mill was required of high precision, which was affected by the actual environment and had certain non-linearity. In order to satisfy the automatic adjustment of control parameters according to the external environment, a fuzzy self-tuning Proportion Integration (PI) controller was designed. Through simulation experiments,it is proved that the system requirements are satisfied by the controller.