The arithmetic of decimation and synchronisation of Base-band signal is a key problem in space-based AIS(Automatic Identification System) processing. Aiming at the defect of the Detection of Striding across Zero and the Early-late Gate, a method to decimate and synchronise the base band signal based on energy measurement was proposed. We divided the base-band signal into segments with the same length according the rate of sampling and the rate of code, and then we calculated the measurement of corresponding sampling point of every segment. Finally we chose the sequence number of the maximum energy measurement as the best decimation and synchronisation sampling point, which has the character of maximum SNR. Numerical performance of the proposed methods performs better than the existing ones. 