A mathematical model for predicting an unstable working condition called cycling of a weight-control pressure relief valve is proposed. The valve is connected by a long pipe to a reservoir used to store liquid cargo. The model numerically solves a transient, compressible, and one-dimensional fluid dynamic equation in the pipe, with boundary conditions given by the valve mechanical equations and reservoir inflow equations. The mechanism of the cycling problem initiation is given by analyzing the gas pressure and velocity oscillation in the pipe. Cycling comes from the small initial vibration of the valve moving part and is caused by the pressure variation delay between both ends of the pipe. Higher pipe length and higher cargo inflow rate are easier to trigger cycling. A criterion for preventing cycling is proposed based on the simulation data, which is the gas pressure loss in the connecting pipe should be lower than 0.06% of the valve set pressure. This criterion is stricter than that used for spring-loaded pressure relief valves.