The invention provides a system and a method for injecting CO2 foam through a wellhead of a mine field. The system comprises a liquid CO2 storing tank, an additive storing tank, a first feeding pump, a second feeding pump, a foam generator and a foam generation degree detecting device, wherein the foam generator is provided with a liquid CO2 inlet, an additive inlet, a foam outlet, and a liquid picking monitoring opening for communicating the inside with the outside of the foam generator; the liquid CO2 storing tank is connected with the liquid CO2 inlet through the first feeding pump; the additive storing tank is connected with the additive inlet through the second feeding pump; the foam outlet of the foam generator is connected with the wellhead of the mine field; the foam generation degree detection device is connected with the liquid picking monitoring opening through a pipeline; valves are arranged on the pipelines which are connected with the liquid CO2 inlet, the additive inlet, the foam outlet and the liquid picking monitoring opening. With the adoption of the system and the method, a stable CO2 foam system can be formed; the technical bottleneck that the foaming state of an injection agent cannot be controlled when working at the mine field can be broken; the technical support is provided for the construction at the CO2 foam driving and CO2 foam profile control sites.