This letter studies the fractal properties of sea clutter, mainly including the box-counting dimensions and multifractal spectra. Those properties of sea clutter with targets and sea clutter without targets are compared and found different, which enables us to develop methods to detect low-observable targets within sea clutter. An integral test is utilized to compare the multifractal spectra of sea clutter under different conditions. In practice, researchers can hardly get enough real data of variable signal-to-clutter ratio (SCR) to get systemic conclusions. Therefore, we construct new sea clutter series from real data with variable SCR. The results show that the detecting methods based on fractal analysis have strong performance of maritime target detection at very low SCR.