Cap assembly for a carbon canister fuel tank

The cap assembly for a carbon canister fuel tank includes an outer cap and an inner cap. There is a cavity with an opening at the inner cap, and the outer cap is above the inner cap covering the opening. There is a lower vent cap in the cavity which divides the cavity into a filling cavity and a volume cavity. There is a through hole in the center of the lower vent cap with a breathing valve which would open automatically when there is air pressure imbalance in the filling cavity and volume cavity. The first vent hole is at the bottom of the volume cavity, while the second vent hole is at the bottom of the filling cavity. The first vent hole, breathing valve and the second vent hole are arranged in sequence to constitute the exhaust channel for the vaporizing oil gas in the fuel tank.