The requirement of consistant flow for steering can be better met with variable displacement vane pump as the steering oil pump of heavy engineering machinery of mining equipment instead of gear pump. This paper described the consistant flow principle of the pump and derived the flow equation of consistant flow pump,As the pump varys in RPM considerably (500~ 2200rpm), the growth rate of its flow is less than l5 %. This characteristic of consistant flow is highly desiral,le for steering oil pump.