Objective： Application of cerebral state index (CSI ) monitoring, to observe and compare the sedative effect of propofol or midazolam during spinal anesthesia, then to investigate the feasibility of CSI in monitoring of sedation. during spinal anesthesia. Mothods: Forty patients received lower abdominal or limb operation under spinal anesthesia were randomly divided into two groups (groupⅠand groupⅡ).Each group owed twenty. The patients of group Ⅰwere sedatived with propofol , Group Ⅱwith midazolam .The changes of OAA/S score , SBP,DBP, BP, SPO2 at the same points were recorded, stopped injection shortly after SPO2<92%. Results: ( 1 ) Before SPO2<92%, OAA/S score all reached grade 1 in group Ⅰ, all reached grade 3 in groupⅡ. There were 6 cases reached grade 2, 2 cases achieved grade 1 in groupⅡ. ( 2 ) The CSI,SBP,DBP, BP, SPO2 were significantly descend with OAA/S score in two groups（p<0.05） ( 3 ) When OAA/S score ≥ 3, the correlation coefficient of OAA/S score and CSI was 0.745 and 0.684 in group Ⅰand Ⅱ(p<0.01, p<0.01). No significant differences between the two groups of correlation coefficient. When OAA/S score <3, the correlation coefficient of OAA/S score and CSI was 0.672 in group Ⅰ( p<0.01 ). Conclusion: CSI can be used to monitor the sedation with propofol or Conscious sedation with midazolam during spinal anesthesia, It can reflect the dynamic changes of sedative depth.