The invention discloses an engine air intake system and an automatic drain valve thereof. The automatic drain valve is arranged in a pipeline between an air intake channel and an intercooler of the engine air intake system, and comprises a valve seat, a valve element and a water inlet runner, wherein the valve seat is provided with a valve cavity, the valve element is arranged in the valve cavity, and condensed water in the air intake channel can flow into the water inlet runner. The valve seat is provided with a drain runner and a valve port which enables the drain runner and the water inlet runner to be communicated. An elastic component is arranged between the valve element and the bottom of the valve cavity. In the process of air intake of the air intake system, the valve element is compressed through air intake pressure, the elastic component moves towards and plugs the valve port. In the process of resetting of the elastic component, the valve element disengages from the valve port. The automatic drain valve can be closed in the process of air intake to maintain leakproofness of the air intake system and can be opened in the process of halt to drain the condensed water in the air intake channel through pressure difference of air pressure and elasticity of the elastic component, therefore, normal work of the air intake system can be maintained, and rust and damage to components and parts in the air intake channel can be avoided.