A principle to determine the orifice area of spool valve with two-stage notches was proposed based on the series parallel effects of fluid flowing in channel of the notches. The formulas to calculate the orifice area were obtained and the corresponding computational program was worked out. Then, the experimental results on the flow characteristic of two-stage notches were analyzed. The results show that the flow coefficient of two-stage notch is the combination of that of each single notch. When the spool displacement was small or near to full open, the flow coefficient increased quickly. At the middle part of displacements, the flow coefficient was about 0.7～0.8. The flow coefficient in flow-out direction was larger than that of flow-in, which was about 0.1. A new concept of orifice area of flow control for defining the flow control function of the orifice was proposed. 