The embodiments of the invention disclose an underwater soil body settlement monitoring system and monitoring method. The system comprises a measuring instrument, a target, a rearview target, an uppercomputer and a supporting structure; the measuring instrument is in communication connection with the upper computer and is used for emitting first optical signals to the target, receiving first reflected optical signal reflected by the target, emitting second optical signals to the rearview target, receiving second reflected optical signals reflected by the rearview target, sending current datainformation to the upper computer according to the received first reflected optical signals and second reflected optical signals; the target is fixedly arranged at the top end of the supporting structure; and the supporting structure is fixed at the bottom of to-be-monitored water. According to the underwater soil body settlement monitoring system and monitoring method provided by the embodimentsof the invention, the supporting structure is used for reflecting underwater settlement to a water surface; an automatic monitoring system is used for monitoring; and therefore, the problems of low precision and low real-time performance in a construction process are solved, the real-time feedback of underwater soil settlement is realized, construction parameters can be conveniently adjusted in time, and the precision level of monitoring data can be improved.