In order to understand the effect of condition and design parameters on trapped oil pressure in external spur-gear pump, based on the geometrical relationship of gear transmission of external spur-gear pump and relative position with relief groove, a new dynamic trapped oil model including equation of trapped oil pressure and dynamic coupled equation of gear pair was derived from the calculations of trapped volume and it’s first derivative and exchange flux from inside to outside of trapped volume and dynamic backlash value. Thereby simulation results of trapped oil pressure were produced and compared with tested results published in a literature. All results indicated the simulation results from dynamic trapped-oil model were greater in value than static trapped-oil model with trapped-oil pressure p2 exceeding p1, besides, all involved parameters discrepantly influenced trapped-oil pressure, accordingly optimization of operate parameter was necessitated. Finally, the validated trapped-oil model was thought to be right and reliable for forecast on trapped-oil pressure as well as vibration analysis along with vibration evaluation and optimization of entire design of gear pump.