A lower fixed beam connecting rod locking type steam airtight cleaning device of a tank car comprises an airtight locking mechanism, a steam inlet pipe for steam filling and a dirt discharge pipe for discharging dirt after steam cleaning. An airtight locking device is in adaptive sealing connection on a tank opening. The steam inlet pipe and the dirt discharge pipe lead to the interior of a groove tank. The airtight locking device comprises a sealing gasket arranged on the end face of the tank opening, a first gland located above the sealing gasket, a lifting hanging frame moving in the height direction of the tank opening, a lifting power element which is fixed to the first gland and connected with the lifting hanging frame and is used for driving the lifting hanging frame to move, and a fixed beam which is basically fixedly connected relative to the tank opening and stretches into the groove tank. Clamp hooks are hinged to the part, stretching into the groove tank, of the fixed beam. A connecting rod mechanism formed by the clamp hooks and the fixed beam together controls opening and closing of the clamp hooks and is driven by the lifting hanging frame. Locking force generated by the locking mechanism in a cleaning system uniformly acts on the end face of the tank opening, and no load burden is caused to other equipment and facilities.