In frequency domain, moving targets with different velocities will affect the different frequency components of sea clutter spectrum. In order to analyze the nonlinear influence, the piecewise fractional Brownian motion is introduced, which builds the relation between different spectrum parts and the fine/coarse scales, i.e. the target echoes at different velocities can affect the self-similarities of sea clutter at different scales in time domain. Based on the real X-band and Sband sea clutter data, this influence mechanism is studied and verified. The results show that the slow-moving target mainly affects the self - similarity of the sea clutter sequence at the coarse time scales, and the fast-moving target mainly affects the self - similarity of the sea clutter sequence at the fine time scales. This conclusion lays the foundation for introducing the fractal theory into the sea clutter spectrum analysis and target detection in frequency domain.