The configuration and the work principle of the flow press hydraulic servo-valve was introduced in this paper, based on this and electromagnetic basic theory, the mechanics analysis of every main parts of the flow press hydraulic servo-valve and the liquid basic theory especially the flow continuity equation, the mathematic model of the flow press sever-valve was built, during the process of building the model, the author took into account the load moment brought by the pair of spray nozzles flow force and the load moment brought by the force couples back pole and the fundamental equation of the force moment motor. On the other hand, the author analyzed the influence of the nonlinear factor to system performance, and the nonlinear factor was valve port overlap length disaccord caused by machining error. The mathematic model of the electro-hydraulic force control system controlled by the flow press servo-valve with valve port error was built. The results of simulation and experiment showed that valve port error badly influenced the system performance, and that valve port error was the main reason of the pressure higher than the supply pressure, which is one of the two cylinder cavities. The simulation curve and the experiment curve were accord compared by system simulation and experiment results, so we can know that the mathematic model of the flow press sever-valve and of the electro-hydraulic force control system controlled by the flow press servo-valve is correct.