Recently, using bistatic high-frequency (HF) radars to monitor the ocean surface attracts much attention. However, only a few works which focus on the extraction of ocean wave information are reported. In this work, a new method for the extraction of the ocean wavenumber spectrum is proposed. This new method assumes that the directional distribution of the ocean waves is known, and then it linearizes and simplifies the theoretical model of the bistatic radar sea echoes. In addition, the proposed method is applied to the simulated sea echo data and the result indicates that the method is effective.