The invention relates to a method for designing the height of an inner tank of an LNG storage tank. The method includes the steps that a liquid unit is constructed according to the motion type of LNG liquid in a cylindrical container; a numerical model is established; modal analysis is carried out on the numerical model through a reduction method to obtain the inherent frequency, the damping ratio and the modal shape of each mode; seismic acceleration spectrums corresponding to different damping ratios are input and combined with modal analysis results for response spectrum analysis, and seismic response results are calculated; response spectrum analysis results are extracted and the sloshing wave height is determined; influencing parameters are determined and sensitivity analysis is carried out on the parameters, so that the sloshing wave height calculation formula is determined through a multi-parameter fitting method according to the influence rule of the parameters; the height of the inner tank of the LNG storage tank is determined according to the calculated sloshing wave height. The height of the inner tank of the LNG storage tank is determined by accurately calculating the sloshing wave height of the liquid in the LNG storage tank under seismic excitation, and the method can be widely applied to LNG storage tank design.