Abstract: The waveguide invariant，which describes the acoustic intensity interference pattern in ocean waveguide with a single scalar parameter, can be used to improve the longitudinal correlation of acoustical field. In this paper, after obtaining the estimation of bearing and waveguide invariant parameter, an approach to passive range estimation based on improvement of the correlation of two horizontal sub-array signal beam outputs using waveguide invariant is derived. By simulation analysis and the validation of the experimental array data at sea with a 360-meter horizontal line array in shallow water, we find that the range estimation error using the derived approach is less than 10% for the source whose range is within 30 kilometers when the SNR at the beamformer output is larger than 6~9dB. However，there is an estimated blind zone at angles near broadside that is about 20 degree for a horizontal array with 360-meter aperture, which can be reduced when the source range decreases， the array aperture increases or the signal frequency increases.