The invention relates to a buoyancy adjustment device and adjustment method for medium and shallow depth submersibles, comprising a pressure-resistant water tank, a pump source is installed at the bottom of the pressure-resistant water tank, and a drainage branch connected to the pump source is arranged outside the pressure-resistant water tank , Water injection branch, pump source includes motor, sea water pump, sea water pump is connected with water inlet filter, outlet is connected with safety valve, and one end of the pipeline protruding from safety valve is open; drainage branch includes water outlet with sea water pump connected to a one-way valve, the one-way valve is connected to the drain switch valve, the drain switch valve is connected to a sea filter, and the sea filter is connected to the external sea water, and the water injection branch includes a water injection filter connected to the bottom of the pressure-resistant water tank, The water inlet end of the water injection filter is connected with a throttling hole, the water inlet end of the throttle hole is connected with a speed regulating valve, the water inlet end of the speed regulating valve is connected with a water injection switch valve, and the water inlet end of the water injection switch valve is connected to the sea. The filter is communicated, and the water injection switch valve is communicated with the drain switch valve, which reduces the number of valve parts in the system.