In a system sealed by dry gas, the Joule-Thomson (JT) effect occurs when the gas flows through the components of filters, valves, orifices and end faces, which may cause the temperature drop of sealing gas, even the appearance of liquid condensation. Generally, the Joule-Thomson effect is represented by Joule-Thomson coefficient. As to the hydrogen, nitrogen, air and carbon dioxide, which are often encountered for the cases of sealing by dry gas, the corresponding Joule-Thomson (JT) coefficients were calculated by four classical equations of state (EOS) of VDW, RK, SRK and PR. Subsequently, those calculated coefficients are compared with the experimental data in the literatures. The JT coefficient curves and Joule-Thomson inversion curves (JTIC) were plotted using the optimal equation of state. As to air and nitrogen through the end faces of dry sealing gas, the gas temperature drops caused by JT effect were calculated by applying the computer program. It shows that the Joule-Thomson effect of real gas, which have important influence on the throttle of dry sealing gas. At room temperature, hydrogen showed exothermic effect, while nitrogen, air and carbon dioxide endothermic (cooling) effect. The corresponding Joule-Thomson (JT) coefficients were calculated by the four classical EOS, the average relative error and maximum relative error of RK equation were the minimum, less than 4% and 10%, respectively. The JT effect of real gas causes large temperature difference in the dry sealing gas, of which the gas pressure more influences on the temperature drop than the gas temperature does. When the pressure is small, the temperature drop by the JT effect can be negligible.