There is a close relationship between the boundary position of the plug flow region and the output damping force when the magnetorheological fluid moves inside the damping channel of the magnetorheological fluid damper.In order to obtain the specific numerical value and variation law of the boundary position of the plug flow region, the shear stress and flow state of the magnetorheological fluid inside the damping channel were analyzed based on RD-8041-1 magnetorheological fluid damper, and the function relationship between the boundary position of the plug flow region and the speed of the piston movement was obtained. For the purpose of verifying the correctness of the obtained functional relationship, on the basis of the mathematical expression of the output damping force of magnetorheological fluid, the output damping force was calculated according to the functional relationship between the boundary position of the plug flow region and the piston speed when the input current was 1 A, and the calculated damping force value was compared with the actual test value. The results show that the calculated damping force is consistent with the measured value, which verifies the correctness of the functional relation between the boundary position of the plug flow region and the piston velocity