Multibeam sounding system is a modern underwater detection technology which has been widely used in the fields of underwater detection. We present the working principle and  characteristics of this system and introduce its application to the monitoring of river sand resources. We also discuss its techniques and data processing process, and calculate the quantity of river sand mining. Through case study in a sand mining segment in the Yangtze River basin, we demonstrated the effectiveness and reliability of applying this system to the quantitative monitoring of regional sand mining.