The particularities of oil suspension formulation can raise the invasive rate of Hirsutella sinensis to host of Hepialidae for the commercialization of artificial cultivation of Cordyceps sinensis. So it is important to develop a high cell viability oil suspension formulation of C. sinensis spawn. According to the characteristics of the oil suspension formulation, MTT assay is adapted and optimized. The result is as follows: reaction time 120 min, reaction temperature 37?C, methylbenzene as extracting agent, and a positive linear correlation established between active cell weights and cell viability. Varieties and concentrations of assistance agents in oil suspension formulation have been selected with the refined MTT assay, and further optimized together with cell concentrations through orthogonal experiment. The optimal combination project was obtained, namely, cell concentration 0.15 g/mL, aluminium stearate 60 mg/mL, and SPAN-80 50 ?L/mL. Results of stability test on the oil suspension formulation indicate that cell viability can maintain above 90% at 4?C after one month.