The disclosure provides a method for estimating the nondirectional wave spectrum from the sea echoes of multiple HF radar frequencies. The method includes: dividing the radar detection area into a plurality of fan-shaped units at an equal range interval and angle interval according to the distance resolution and the angular resolution of an HF radar; obtaining the Doppler spectrum from the sea echo of a single radar frequency at a fan-shaped unit by performing the first fast Fourier transform (FFT) in distance dimension, the second FFT in Doppler frequency dimension and the digital beamforming; extracting the positive first-order peak and the negative first-order peak from the aforementioned Doppler spectrum by the peak-searching method; and selecting the stronger first-order peak σR(1)(ω); dividing the second-order spectrum on the stronger first-order peak side into an inner second-order spectrum and an outer second-order spectrum.