The invention discloses an air injection device for a subsurface drip irrigation system. A subsurface drip irrigation system is started, an air inlet gate valve is switched on, and water with pressure from the head of the subsurface drip irrigation system is injected out at a high speed from the injection orifice of an ejector nozzle to form negative pressure in an air injection cavity; under theaction of the negative pressure, air enters the air injection cavity from an air filter, the air inlet gate valve, a check valve and an air inlet pipe; the air and high-speed flow ejected out of the ejector nozzle enter a primary oscillation cavity via a round throat path of which the diameter is d1 to carry out primary vortex oscillation stirring 'aerification'; then, the mixture enters a secondary oscillation cavity via a round throat path of which the diameter is d2 to carry out secondary vortex oscillation stirring 'aerification'; and finally, the mixture enters the subsurface drip irrigation system via a round throat path of which the diameter is d3 and a gradual-change runner. The air injection device has the advantages of good air injection effect, high aerification rate and low input and operation cost, is simple and durable, does not need additional pressurized driving devices and is especially favorable for improving the anti-blockage performance of the injection device.