To research the dynamic flow field characteristics of main valve during the safety valve opening period, the user-defined function method and Fluent dynamic mesh method with virtual technic are used for initial fluid-structure interaction numerical simulation. The pressure force of diaphragm is obtained through solving the air-capacitor filling and exhausting process equation, while valve speed and displacement are solved through the mass-spring-damping system dynamic equation. Results show that flow filed becomes more complex and unstable with larger valve opening degree. To analyze the factors affecting the dynamic instability of flow field, the relationships between the opening degree and flow changes of inlet, outlet and throat are researched, also pressure changes of the valve and diaphragm are studied.