Based on Van der Waals equation and theory of fluid mechanics, the liquid real-time continuous leakage model of horizontal liquid ammonia tank was established considering the changes of tank pressure and liquid surface area. Mathematical modeling of a horizontal liquid ammonia tank in a refrigeration company was performed by this model, and the calculation results were compared with PHAST simulation results. The results showed that the decreasing range of liquid level height h grew slowly and then increased quickly, the decreasing range of liquid leakage mass flow rate Qm and liquid leakage rate v and the increasing range of liquid leakage mass m decrease slowly. At the beginning of leakage, Qm and v were the maximum values, at the end of leakage, m was the maximum values. When the diameters of leakage hole were 5, 30 and 100 mm, leakage time t were 29884.027, 837.289, 77.550 s, Qm(max) were 0.552, 19.913 and 221.160 kg/s, v (max) was 46.733 m/s, m(max) were 10255.649, 10339.923 and 10572.760 kg, respectively. The deviation between the calculation results of the model and the PHAST simulation results was less than 24%. From the analysis of parameter variation and risk emergency, the model had the certain applicability for the theoretical calculation of liquid leakage in horizontal liquid ammonia tank.