In sea clutter background, spaceborne or airborne surveillance radar with a high-speed moving platform is confronted with the problem of clutter spreading. This poses a considerable challenge for maritime radar systems when detecting slowly moving targets of low signal-to-noise ratio. However, the space-time decorrelation characteristics caused by the internal motions of sea clutter under different sea conditions are mostly neglected in the current detecting techniques, which go against the reliable parameter selection in the clutter suppression process. An innovative multi-channel space-time decorrelation analysis method based on sea clutter is proposed. First, the channel decorrelation factors caused by the channel amplitude-phase errors and the noise are calculated by fusing the calibration signal with echo data. Then, the temporal decorrelation curve caused by the internal motions of sea clutter is obtained by fitting the peak points of multi-channel correlation coefficient curve based on the multi-channel phase center. Finally, the independent spatial decorrelation curve is derived through calculating the multi-channel correlation coefficients of different range bins. Due to the utilization of space-time characteristics decomposition method for sea clutter, the space-time coupling problem can be solved effectively. The experimental results based on the measured data prove the effectiveness of the proposed method.