Abstract: Based on the principle of hydraulic control rod driving system(HCRDS), the paper gives a new scram method, which makes the inside pressure of step cylinder discharge, to solve the scram problem of HCRDS in control rod hang up accident and inversion conditions, and some experiments were caried out with the 1∶1 experimental loop of the HCRDS for the 200 MW nuclear heating reactor(NHR200). The results show that the control rod falls much quickly than that in normal condition. The differential pressure between the outside and inside of step cylinder can reach a large value after the inside pressure of step cylinder discharge. In other word, the hydraulic control rod, which can overcome certain frictional resistance in control rod hang up accident of its gravity in inversion condition, can insert the nuclear reactor core.