The invention relates to a device and a method for testing natural gas, in particular to a device and a method for testing gas storage density of a natural gas hydrate. The device comprises a hydrate reaction system, a gas injection and circulation system, a liquid injection and circulation system, a pressure-stabilizing buffer system and a differential pressure transmitter, wherein gas in upper space of a pressure-stabilizing buffer tank is connected with the differential pressure transmitter through a gas communication valve, a liquid in the lower part of the pressure-stabilizing buffer tank is connected with the differential pressure transmitter through a liquid communication valve, and a balance valve is mounted between the gas communication valve and the liquid communication valve; the pressure-stabilizing buffer tank is connected with a water tank through a water supplementing valve and connected with a reaction kettle through a gas supplementing valve. According to the device and the method, influence of temperature and pressure conditions with dynamic fluctuation change in an experiment process on gas flow measurement is eliminated, consistency of the temperature and pressure conditions at the initial moment and at the end moment of an experiment is guaranteed, and on the basis that the water supplementing volume is equivalent to the volume of consumed gas, accumulated errors in the existing measurement process with a flowmeter are reduced.