A novel target depth estimation method based on normal mode intensity match is proposed for shallow water environment using horizontal array to overcome the performance degradation observed in conventional approaches under seabed parameters mismatch condition. First, horizontal wavenumbers and normal mode intensities are estimated through wavenumber domain beamforming. Second, modal function of normal mode inversion is performed by solving the modal function characteristic equation through finite difference method. Third, the match degree between inverted and estimated normal mode intensities is evaluated to estimate target depth. Numerical simulation results demonstrate that the proposed method can achieve accurate target depth estimation in shallow water scenarios without knowledge of seabed parameters. Furthermore performance of the method is analyzed under varying conditions including different seabed parameters, array apertures and source frequencies. The results reveal three conclusions: (1) mismatch of seabed parameters has no impact on the method; (2) effective performance of all depth source estimation requires not less than 128 array elements, 50-150Hz frequency band range and the signal-to-noise radio in the element on a horizontal line array exceeds -10dB; (3) the method has robust performance against sound speed profile mismatch. Finally, the feasibility of the proposed method is validated through experimental data received by a horizontal towed 77-elements array during a shallow-water sea trial at the South China Sea.