This paper aims to scientifically analyze the impact of the pure liquid leakage accident and explore the leakage calculation of pure liquid ammonia from the liquid ammonia storage tank. Based on Van der Waals equation and fluid mechanics method, the liquid leakage model of liquid ammonia tank was established considering the tank pressure change of liquid leakage in liquid ammonia tank (vertical tank, horizontal tank and spherical tank) and the characteristics of different types of tank structure features resulting in the change of liquid surface area. Numerical simulation of liquid leakage for three liquid ammonia tanks was performed by the model, which were then compared with PHAST simulation results.The results show that the decreased range of liquid level height hv,s,h decreases slowly and then increases fastly, the decreased range of liquid leakage mass flow rate Qm-v,s,h, as well as the increased range of liquid leakage mass mv,s,h, decreases gradually and slowly. At the beginning of the leakage, the Qm-v,s,h reached the maximum values of 0.555, 0.553, and 0.552 kg/s; at the end of leakage when tank leaks, mv,s,h reached the maximum values of 11.245 846, 11.084 621, and 7.730 932 t, respectively. Model calculation results are similar to PHAST simulation results, the deviation of Qm-v,s,h(max) was small and in the range of 16.577%～16.667%, while the deviation of mv,s,h was relatively large, and the deviation of leakage liquid mass in vertical tank, spherical tank and horizontal tank were 4.565%, 5.764% and 17.630% respectively. From the analysis of parameters' variation, these model is suitable for calculation of liquid leakage in liquid ammonia tank and its risk analysis.