Abstract: With the continuous development of underwater automatic monitoring technology, the real-time data transmission efficiency and measurement accuracy are also increasingly demanding. However, there are still serious deficiencies in high-quality lossless data compression and transmission technology. In this paper, a data compression preprocessing method based on target characteristics is proposed by using the side-scan sonar ship draft automatic monitoring system established on the Xijiang channel in Guangxi. According to the hull structure and the imaging characteristics in the acoustic image of bulk carrier, the target signal is identified and extracted in time and space dimensions to eliminate irrelevant redundant data before compressing the data. This preprocessing method can not only greatly improve the data compression ratio, improve the transmission efficiency and save storage space, but also ensure no loss the key feature information of the target used for calculation. This method also provides a new idea for further breaking the compression ratio limitation of lossless data compression.