The invention relates to a back-injected clay slurry of a composite lining tunnel and an injection method thereof. The clay slurry is prepared by mixing a component A and a component B, wherein the component A comprises the following components in parts by weight: 80-130 parts of water, 50-80 parts of clay, 30-50 parts of cement, 0.5-10 parts of clay accelerator and 0.05-3 parts of plasticizing agent, the component B comprises the following components in parts by weight: 80-120 parts of water, 80-120 parts of water glass and 0.5-10 parts of retarder; and the mixing mass ratio of the component A and the component B is 100:(30-60). In the method, the clay is pre-soaked before injection so as to improve the hydration of the clay, and the component A is treated by using a high speed and low speed combined binary stirring method to improve the dispersity and the slurrying speed of the clay slurry. Cavities between a lining and a surrounding rock can be filled with lower cost by using the method so as to improve the stability of the surrounding rock and protect the lining, and the method almost does not need secondary injection and is suitable for the back injection of the composite lining tunnel.