The invention relates to the field of coal mine safety, in particular to a warning grading method and a warning method for spontaneous combustion of coal. The coal is prone to spontaneous combustion,and the warning for spontaneous combustion of coal is divided into six grades, namely, grey warning, blue warning, yellow warning, orange warning, red warning and black warning. The warning method includes the following steps: collecting O2, CO, CO2, CH4, C2H4 and C2H6 concentration data; judging the warning grade, and setting a warning threshold as R0, wherein the warning threshold of coal spontaneous combustion can be expressed by the following mathematical formula: R0={[O2]>18% union [CO]>50ppm} intersecting {[O2] belongs to (15%, 18%) union [CO]>100ppm} intersecting {[O2] belongs to (12%,15%) union [CO]>150ppm} intersecting {[O2]<12% union [CO]>200ppm}; and issuing a warning and taking measures according to different thresholds. The spontaneous combustion process of coal can be indicated more accurately and sensitively, and higher preventability is achieved.