Installed on the top of tank car body to seal the tank mouth, which is completely sealed with the outside to ensure the anti-overflow and anti-explosion function of tank car when tilting. When the pressure inside and outside the tank changes, the built-in breather valve installed on the large cover can effectively adjust the pressure to ensure the pressure balance inside and outside the tank, when the tank internal pressure is too high, the built-in breather valve can not meet the discharge requirements, the gas in the tank can overcome the spring pressure on small cover to open the small manhole cover, which will release excessive high pressure quickly to ensure safety. At the same time, the installation port of anti-overflow probe rod, the installation port of oil and gas recovery valve and small oil hole are reserved on the large cover for easy installation.