Abstract: During the testing of deep water oil and gas wells, gas hydrate is prone to block downhole safety valves. To prevent it from happening, a method for determining the reasonable setting depth of safety valve was studied. The gas hydrate phase of equilibrium micro-test device was used to simulate the phase transition process of multi-component gas hydrates under various formation water salinities in the laboratory and to obtain the influencing law of temperature and pressure on the phase equilibrium of gas hydrate. The effects of gas composition, water depth, the geothermal gradient and wellhead pressure on the formation of gas hydrate were analyzed to predict the formation area of gas hydrate, and the method in determining the minimum setting depth of safety valve was obtained from the aspects of safety and cost. Studies suggest that all the factors including gas composition, water depth, geothermal gradient, and wellhead pressure could affect the setting depth of safety valve, and the increased contents of ethane, propane and butane in the produced gas are more likely to form gas hydrates. In addition, the setting depth of the safety valve will be further lower as deeper water depth, smaller geothermal gradient, higher wellhead pressure, and larger gas hydrate formation area. The results of this study could provide a reference for determining the setting depth of test safety valve in deep water oil and gas wells.