The purpose of this work was to guide the design of the suspended control rod in a sodium-cooled fast reactor. Retaining the dimensions and main flow characteristics of the prototype device, a hydraulic model of the passive shutdown device was designed. An experiment was carried out on the critical flow of the upper working position and the lower position (accident condition). The critical flow rate maintaining a stable suspension in the upper working position and hydraulic self-tightening in the lower working position was determined. The study found that the critical flow of the control rod in the lower working position was obviously affected by the circumferential angle; the force that causes the control rod to incline was inevitable and the key position that affects critical working conditions was pointed out. The study also found that the working fluid temperature had little impact on the critical working conditions.