Abstract: The calculation of the total calorific value of the LNG trade measurement requires the use of the natural gas composition analysis data. But when the sampling in the unloading process fails, the prediction method can be used to calculate the unloading port natural gas composition. The prediction method is to establish the relationship of liquid and gas phase based on the boiling point of natural gas. According to the evaporation rate, unloading port natural gas composition is obtained by multiple iteration to loading port natural gas composition. Through comparative analysis, it is concluded that the deviations of natural gas composition, the calorific value per unit volume and LNG density are small between the predicted method and the actual sampling analysis. The prediction method has reference value for LNG terminal loading and unloading process sampling analysis, as well as buyers and sellers trade settlement.