The structure of thrust block carriage that adopting mechanical adjustment pass was introduced, Focus on more change of pass of rotary expanding mill adopting the mechanical adjustment pass during rolling process, an thrust block carriage that adopting hydraulic adjustment pass was designed, and an electric-hydraulic servo control system was designed according to its structure, in order to compensate the pass changes caused by temperature change of rollers and plug, so that enhancing wall thickness precision of steel pipe. The composition and principle of the electric-hydraulic servo system was introduced, and the electric-hydraulic servo system was modeled and simulated based on ITI-SimulationX, the system was simulated and analyzed, the result of the simulation show the design can meet with the dynamic adjustment request of pass during rolling. The reliability of the electric-hydraulic servo system was verified by measurement record of steel pipes.