The invention relates to an engineering machinery air storage tank automatic drainage device and an automatic drainage control method and aims to solve the problem that the drainage operation is inconvenient in a conventional engineering machinery air storage tank. According to the air storage tank automatic drainage device, a sensor acquires mechanical parameters and transmits the mechanical parameters to a controller; the controller receives a parameter signal, makes judgment that whether water needs to be drained or not and transmits a draining electric signal when the water needs to be drained; a drainage valve is arranged on the air storage tank to receive the draining electric signal which is sent by the controller and executes the drainage action. The sensor is used for checking to acquire the drainage parameter indicating the drainage action, the controller calculates and controls whether the water needs to be drained through drainage valve, automatic drainage is realized, the drainage timeliness and reliability can be well guaranteed, manual leakage and drainage is eliminated, dried gas in the air storage tank is fundamentally guaranteed, and the stability and reliability of a braking system are greatly improved.