Abstract: 【Objective】 Natural fire disturbance surveys were conducted in Kanas Nature Reserve and the external attribute factors of fire scar in Larix sibirica Ledeb. were used to quantify the fire severity of natural fire disturbance of virgin forest in Kanas Nature Reserve. 【Method】 Through field investigation combined with cluster analysis, the fire severity could be quantified into high-, moderate- and low-severity, and each level had its corresponding token factors range, which could be used as the basis for the classification of forest fire intensity in that Nature Reserve. 【Result】 In Kanas taiga, plots of high-severity accounted for 37.4%; moderately burned plots accounted for 30.7% and low-severity accounted for 31.8%. The fire history could be attributed to 63 years in between 1880 and 2000. Fire severity levels were no equal before 1990. After 1990 the times of fire decreased and the fire severity was dominated by the moderate and low levels. 【Conclusion】 Conclusions were that the natural fires in Kanas Nature Reserve were mainly dominated by the high- and moderate- fire severity. And the fire severity was affected by terrain factors, per-fire stands composition and forest fire management.