The utility model provides an underwater sediment sampling robot which comprises a main cabin body, and a propeller and a sampler which are arranged on the main cabin body, wherein a counter weight and a gas cylinder are arranged on the main cabin body; the gas cylinder is positioned outside the main cabin body; an exhaust valve is arranged on the gas cylinder; the counter weight is arranged on the lower part of the main cabin body and is close to the gravity center position close to the main cabin body; the gas cylinder is arranged on the outer side of the top of the main cabin body; the exhaust valve is arranged on the top of the gas cylinder; and a water inlet valve is arranged at the bottom of the gas cylinder. According to the sampling robot disclosed by the utility model, an operator can be controlled to search the optimal sampling position in a zero buoyancy state, the disturbance-free sampling function is realized by changing the net buoyancy so as to close a sampling pipe, the device is maintained in a horizontal attitude by controlling the flow of the exhaust valve and the water inlet valve, and the attitude is conveniently adjusted by virtue of the water inlet valve.