Disclosed is a combined heat exchange system of flash tank and preheater for laterite nickel ore leaching. The system comprises a high-pressure reactor, preheating towers, and flash tanks. The preheating tower is connected to the feed inlet of the high-pressure reactor. The flash tank is connected to the discharge outlet of the high-pressure reactor and is also connected to the preheating tower through a steam pipe. Inside the flash tank, there is a pressure relief chamber and a buffering mechanism. The buffering mechanism comprises a buffering component and a connecting structure that are interconnected. The buffering component features a central bulge with a buffering tank incorporated. The highest point of the central bulge is positioned higher than the lowest point of the buffering tank, and the surface of the central bulge connects seamlessly with the surface of the buffering tank. Fluid falling from the pressure relief chamber can pass over the surface of the central bulge and slide out from one end of the buffering tank to the other. In this disclosure, the buffering component of the pressure relief chamber is equipped with a central bulge and a buffering tank, materials falling from the pressure relief chamber can sequentially pass over the central bulge and slide from one end of the buffering tank to the other, then slide upwards out of the tank, achieving a buffering effect.