Gas flow switching system and gas flowmeter

The utility model discloses a gas flow switching system and a gas flowmeter, the gas flow switching system comprises an upper cavity and a lower cavity, the upper cavity and the lower cavity are respectively communicated with a gas outlet nozzle and a gas inlet nozzle, the upper cavity and the lower cavity are communicated through a communication cavity, and an opening and closing piece is arranged in the upper cavity; the lower cavity is communicated with an air inlet pipeline, an air inlet reversing switching valve used for switching large-flow air inlet and small-flow air inlet is arranged on the air inlet pipeline, the air inlet reversing switching valve is communicated with the bottom of the large piston cylinder and the bottom of the small piston cylinder respectively, and a small piston and a large piston are arranged in the small piston cylinder and the large piston cylinder respectively in a clearance sealing mode. The upper cavity is communicated with an air outlet pipeline, the air outlet pipeline