Abstract: Objective To analyze the prethrombotic state in patients with severe obstructive sleep apnea syndrome（OSAS）. Methods A total of 105 patients,enrolled in this study,were divided into severe OSAS group（n=45） and control group（n=60）.The sleep was monitored and the sleep-related prethrombotic state was detected in patients of the severe OSAS and control groups. Results The level of haematocrit（HCT） was higher,the activated partial thromboplastin time（APTT） was longer,the number of fibroblasts（FIB） and D-dimer was greater,the levels of antithrombokinase III（AT-III） compounds reflecting the coagulation and anticoagulation function and tissue plasminogen activator（t-PA） reflecting the activity of fibrinolysis syste were lower,the plasminogen activator inhibitor（PAI） was higher,respectively,in the severe OSAS group than in the control group. Conclusion Prethrombotic state,a risk factor for thromboembolic cardiovascular and cerebrovascular disease,can be found in patients with severe OSAS.