Evidences have shown that the magnitude of antihypertensive induced by CPAP were correlated with apnea/hypopnea index (AHI).The severe OSA patients benefit from CPAP treatment was greater than mild-moderate patients. But whether or not all severe OSA patients with hypertension obtain obvious antihypertensive effect from CPAP therapy? In this study, one hundred and fifty subjects with severe OSA were recruited and categorized into three groups: Group l: systolic blood pressure index was less than 30% of AHI; Group 2: systolic blood pressure index was less than 60% but more than 30%; Group 3: systolic blood pressure index is more than 60% of AHI. Nocturnal dynamic BP changes was monitored continuously during polysomnographic examination via pulse transfer time(PTT) based method. The investigator would compare the effect of CPAP treatment on awake and sleep BP level at the first night and 2 weeks therapy among three groups. Moreover, whether or not the sympathetic-parasympathetic nerve balance and the renin-angiotensin-aldosterone system are different in three groups would also be evaluated. The main purpose of study is to confirm the OSA with hypertension and OSA secondary hypertension are two different concepts. Basis for the clinical treatment, the former, CPAP might have no effect or only a minor decrease in arterial blood pressure, while the latter CPAP treatment might achieve significant antihypertensive effect.