The pressure test of the large LNG storage tank is performed by hydraulic pressure. After the test, the LNG storage tank needs to be dried and replaced. In engineering, the method of continuous nitrogen blowing is often used for drying and replacement, which can result in high costs, high nitrogen consumption, and long construction periods. In addition, the imperfect theoretical calculation can also lead to errors in the calculation of LNG tank drying and replacement. To reasonably save and accurately estimate the amount of nitrogen used for drying and replacement, two methods of LNG tank drying and replacement, continuous blowing and pressure expansion, are introduced and analyzed based on the actual project situation. At the same time, the influence of parameters such as oxygen content and dew point is considered to derive a formula for calculating nitrogen usage and develop relevant calculation software. Comparative analysis with the specific construction situation on site shows that the proposed comprehensive calculation method for drying and replacement can quickly and accurately calculate nitrogen usage and operation period. The method also can guide the theoretical calculation and construction work related to LNG tank drying and replacement.