The invention provides a reversing valve applied to an underground fluid sampling process. The reversing valve comprises a valve body top cover, a valve body, a valve element, a spring and a valve body bottom cover. A valve body gas inlet connected with a high-pressure gas source is formed in the middle of the valve body top cover. A valve body fluid inlet and a valve body fluid outlet are formed in the middle of the valve body. An L-shaped through hole is formed in the top end of the valve element, and a transverse straight-through hole is formed in the middle of the valve element. A spring is mounted on the lower portion of the valve element and the protrusion portion of the inner side of the valve body bottom cover. In a natural state, the valve body fluid inlet, the straight-through hole of the valve element and the valve body fluid outlet communicate to form a passageway, and outside environment fluid naturally flows in. After high-pressure gas enters the L-shaped through hole through the valve body gas inlet, the valve element is pushed to move downwards, finally a valve element L-shaped through hole gas outlet communicates with the valve body fluid outlet, and the outside fluid cannot flow in. According to the reversing valve, a gas channel and a fluid channel are separated, the fluid channel is in a normally opened state, underground water can freely flow into a sampling device, no pressure sensitive element is arranged, and no start pressure is generated.