Pressure safety valves (PSVs) are designed to provide ultimate protection for pressure systems. For design or optimization purpose, it is necessary to study the lift force exerted on the valve disk. Under special valve configuration, the lift force shows discontinuity with the increment of valve opening, which is an undesirable phenomenon because it may be the cause of some serious problems, such as unstable valve operations. To study their characteristics in depth, experimental investigations are conducted in this paper. For lift force measurement, an experimental rig was specially designed and built. A series of force tests were carried out, and the result indicates that two variables, namely, the position of the adjustment ring and the thickness of the valve nozzle, have great influence on lift force; under certain valve configurations, the lift force has a discontinuous performance; for different configurations, degrees and positions of force discontinuities are very different; lift forces are linear functions of the inlet pressures, and the slopes of the force changes are similar for different valve openings.