Abstract: The three dimensional flow field of the step cylinder in Hydraulic Control Rod Driving System(HCRDS) is calculated and analyzed with the computational fluid (dynamics) program CFX5 in detail. The results indicate that the flow velocity is lower and the pressure is high in majority of the step cylinder, the distinct variations of the flow field occur in the threshold of the step cylinder at the bottom and the abrupt transition (sections), the holes in the inner and outer tubes and the exhaust hole.