Abstract: During the stress of head up tilt plus lower body negative pressure,the subjects' cardiovascular responses might be divided into four characteristics: vascular type,mixed type,cardiac type and less regulating (lower efficient) type.The regulating characteristics of vascular type mainly showed an increase of total peripheral resistance during stress.The mixed type had an obvious increase of both heart rate and total peripheral resistence.The cardiac type appeared obvious in the increase of heart rate.The less regulating type couldn't be sufficiently regulated in both heart rate and total peripheral resistance.The subjects of vascular type was showed the best tolerance of the stress,then those of the mixed,and then those of the cardiac.The subject of less regulating type showed the worst tolerance.During bed rest,the subjects' feeling and the objective examinations also showed the vascular and the mixed types were better than the other two types.Before and after bed rest,the characteristics of reaction to this stress were similar intraindivi-dually.Data might be treated with the concept of fuzzy sets.The difference among different characteristics of cardiovascular reactions might be quantitatively compared.