Abstract: Based on five seam coal samples from Shendong Mining Area as the study objects, the spontaneous combustion mark gases were optimized using the temperature programming experiment. Meanwhile, the CO gas production rate under different temperature conditions was obtained. The CO and O2 volume fraction of return air corner and goaf was tested at the field, and the width of heat dissipation zone and oxidation zone was also divided. The computational model of CO index critical value in return air corner was established. The study results showed that the mark gas of coal spontaneous combustion in Shendong Mining Area was CO, C2H4 and C2H2 which can be used as the sign for the coal spontaneous combustion initial stage, the acceleration stage and the intense oxidation stage. The critical value of the CO index in return corner was 90×10-6. When the CO volume fraction reached 350×10-6, the gob coal had reached the critical temperature which was 60 ℃ to 70 ℃.