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The echo scattered from a slow-moving weak target on sea surface is nonstationary due to the influence of waves. Time-frequency distributions are good tools to analyze it. In this paper, we propose a method for detecting slow-moving weak targets in sea clutter, which is based on time-frequency iteration decomposition. This method consists of three stages. First, we present a fast signal synthesis method (FSSM) based on eigenvalue decomposition. The FSSM can synthesize a signal faster and more accurately from the Wigner distribution (WD). Then, we present a signal iteration decomposition method (IDM) from the masked WD and the FSSM. By the IDM, the small component of a signal can be obtained, even when it is very close to a large component in the time-frequency plane. Finally, the proposed method results from the IDM and two criteria. Here the two criteria are defined to select the target signal. The proposed method is evaluated by X-band sea echo with a weak simulated target or a real target. The results demonstrate that it not only detects the slow-moving weak target but also shows its instantaneous state.