In the process of in-situ exploitation of oil shale, the production well communicates with the surrounding environment, and the oil generated in situ easily penetrates into the surrounding formation to cause environmental pollution. At present, there is no economical in-situ sealing technology to solve this problem. Based on the principle of grouting closure, this paper proposes to use a non-polluting and low-cost ultra-fine cement grouting formula to seal the mining area. After optimization of a series of formulas, it is finally determined that the optimum formula is made up with water-cement ratio of 0.8, micro-silica powder content of 6%, bentonite content of 2%, and polycarboxylate water-reducing agent content of 0.4%. The performance of the formula basically meets the engineering requirements.