The invention discloses an air-controlled groundwater depth-fixed sampler, which relates to the field of groundwater sampling and includes a mounting platform, a servo motor, an air pump, and a sampling mechanism. Both the servo motor and the air pump are fixedly installed on the top side wall of the mounting platform. The top side wall of the installation platform is fixedly provided with a bracket, the side wall of the bracket is rotatably connected with a reel, the side wall of the reel is wound with a sling, and the end of the sling is fixedly connected with the sampling mechanism, The sampling mechanism is lowered to the required depth through the sling, and the air pump is controlled to inflate and deflate the two sampling tubes on the side wall of the lower end of the mounting seat successively, so that the inside of the two sampling tubes can be filled with water successively, and then in a certain After the first water sampling is completed, the sampling mechanism can be retracted to the second depth to carry out the second water sampling work, so as to realize the function of multi-layer sampling by setting out the line at one time, without the need to monitor the sampling mechanism multiple times. Take it back in the well, and put it down again, which simplifies the process and is more convenient.