The invention provides a method for distinguishing gaseous water dispersing mode in bituminous mortar, and belongs to the field of traffic pavement materials. The method comprises the following steps:manufacturing a thin-sheet type bituminous mortar test part of which the diameter-height ratio is beyond 10: 1; performing gaseous water absorbing test under constant temperature T and different relative humidity phi conditions; calculating equilibrium absorption capacity of the test part in each humidity; then performing gaseous water dispersing test under a constant-temperature and constant-humidity condition, wherein the constant temperature is identified as T; testing the overall mass of the test part device in different time until the stable stage that mass varies at constant speed as time goes one is achieved; then calculating the data obtained from the absorbing test and the dispersing test; and calculating the proportion of the diffusion flux of different dispersing modes in the total diffusion flux. With the adoption of the method, the problem that the gaseous water dispersing mode in the bituminous mortar cannot be distinguished by the prior art can be solved; and the methodis applicable to bituminous mortar damage mechanism study.