Abstract: The nutritional composition, physicochemical indexes and harmful substance contents of flaxseed oil during frying process under conventional frying condition were studied in this paper, and its frying performance was comprehensively analyzed by principal component analysis. The results showed that the unsaturation of flaxseed oil without frying was 81.77%. The content of α-linolenic acid was 47.50%, which was a kind of high nutritional value edible oil. During the 60 h (5 d)170 ℃ continuous frying, the color, acid value, peroxide value, refractive index, carbonyl value, polar components, tocopherol content and fatty acid composition of flaxseed oil changed significantly (P<0.05) , and there was no Benzo (α) pyrene produced in the whole frying process. The content of five main fatty acids (α-linolenic acid, oleic acid, linoleic acid, palmitic acid and stearic acid) all showed downward trend, and the content of five saturated fatty acids which were below C17:0 (octanoic acid, capric acid, lauric acid, myristic acid and palmitic acid) increased gradually with frying time extended, the content of other fatty acids decreased. The analysis of frying performance showed that the acid value of flaxseed oil after 35 h frying had exceeded the national limit, and other indexes were within the national limits. Frying for 30 h had basically reached the frying limit of flaxseed oil. It would not be recommended to use flaxseed oil for frying beyond the limit, it would be a healthy frying oil within 30 h.