This paper considers the nonlinear transformation of irregular waves propagating over a steep slope (1:10). Two experimental cases of random waves mechanically generated by JONSWAP spectra are used for this purpose. The results indicate that the wave heights in two cases obey the Rayleigh distribution both at the offshore location and on top of the slope, but, in the shoaling region, they behave different characteristic. The nonlinear phase coupling occurred within the irregular waves is investigated by the wavelet-based bicoherence. The bicoherence spectra reflect that, in the shoaling region, with the decrease of water depth, the number of frequency modes participated in the phase coupling increase, and the degree of the phase coupling enhances. In addition, the summed bicoherence shows that the frequency mode related to the local strongest nonlinear interactions shifts to higher harmonics with the decrease of water depth. This paper also investigates how the low frequency energy increases by the Fourier spectra, bicoherence spectra and summed bicoherence.