The utility model discloses a detachable slow flow water body fish in-situ observation experimental device. A box body is a cube formed by fixedly connecting eight square steel tubes and four square steel tubes through screws. Mesh cloth is located on the inner side of the box body. In the using process, auxiliary polyethylene ropes are pulled out of the four corners of the top end of the box body respectively, then tied above the box body and connected with a main polyethylene rope, and the main polyethylene rope is fixed to a fixing object after the length of the main polyethylene rope is adjusted. A fish taking and observing window is formed in the upper layer mesh cloth of the box body, one end of a lifting rope of the fish taking and observing window is fixed to the outermost end of an observation window switch, the other end of the lifting rope of the fish taking and observing window is fixed to the main polyethylene rope, and the fish taking and observing window is opened and closed by lifting the lifting rope. The detachable slow flow water body fish in-situ observation experimental device is simple in structure, and can be carried after being detached, the mesh cloth can be replaced flexibly according to the specifications of experimental fish, and the detachable slow flow water body fish in-situ observation experimental device is convenient to use, suitable for in-situ observation of fish in a slow flow water body and capable of directly and simply reflecting the influence of the experimental water body on the fish.