Gear pump has attracted much attention due to the advantages of simple structure, wide speed range, impact resistance, strong suction, yet there are also drawbacks such as radial force imbalance, flow pulsation, excessive vibration and noise. The purpose of this paper is to study the coupling laws between the internal flow field and the vibration of the gear pump under high speed and heavy load working condition, so as to evaluated the service performance of the gear pump under bad working conditions. Especially, the formation mechanism of oil film within the gear contact region was revealed. The finite element method was used to simulate the fluid-structure coupling behavior of the outer-tooth gear pump, and the operating status of the internal fluid-structure coupling field of the gear pump under different speed, load and lubrication conditions were obtained. The impacts of the gear speed, load, inlet oil pressure and oil parameters on the key performance parameters of the gear pump were illustrated. From the analyses it can be seen that  increasing the speed will increase the vibration amplitude, but at the same time, it will improve the suction effect, thereby inhibit the vibration of the gear. A proper load is beneficial to the formation of lubricating oil film, thus reducing the friction heat in the gear contact area. It is not recommended to improve the oil film thickness by increasing the inlet oil pressure, yet the friction and vibration can be alleviated by changing the oil parameters appropriately.