Abstract: A method of calculating the waveguide invariant based on the frequency-shift compensation method,which is used to improve the longitudinal correlations of acoustic fields,is developed in this paper. Processing of the signals received by only two hydrophones with a certain longitudinal separation is needed in the method,which is easy to be carried out. The method calculates the waveguide invariant by searching it in a wide range,and select the optimal value that can maximize the frequency-shift compensated spatial correlation coefficient of the two signals. Results from the shallow water acoustic experiment show that highly accurate estimation of the waveguide invariant can be achieved by using this method. The dispersive properties of the measured waveguide invariants show that frequency can influence the value of the waveguide invariant of the shallow water acoustic field to a certain degree under the condition of low frequency,so the waveguide invariant is not always invariant.