Underground water fixed-depth sampling device

The utility model provides an underground water fixed-depth sampling device, which relates to the technical field of underground water sampling and monitoring, solves the problems of non-representativeness in sampling and large well washing volume in the prior art, and comprises an upper barrier, a centralizer, a lower barrier, a sampling pump component and a connecting pipe, a water passing hole is formed in the pipe wall of the centralizer of the linear hollow pipeline; the upper-layer blocking device and the lower-layer blocking device are both coaxial with the centralizer and are fixedly connected with the top end and the bottom end of the centralizer respectively; a lower-layer gas pipe of the lower-layer barrier penetrates through the centralizer and the upper-layer barrier, the top end of the lower-layer gas pipe is positioned above the upper-layer barrier, and the pipe wall of the lower-layer gas pipe is hermetically connected with the upper-layer barrier; the sampling pump assembly is fixedly arrange