In order to improve the effect of demulsification and dehydration of demulsifier，broaden the scope of application，the structure of polyether polyols demulsifier EG-01 and EG-02 was modified by methacrylic acid，the preparation condition of demulsifier was optimized，and the effect of demulsification and dehydration of modified demulsifier on crude oil in Shengli oilfield，Sudan oilfield and Lvda oilfield was studied. The results showed that methylacrylic acid modified polyether synthetic demulsifiers NG-01 and NG-02 had the best dehydration effect under optimum synthetic conditions，6 h reaction time，110℃ reaction temperature，7∶2∶1 mass ratio of methacrylic acid，polyether polyol and tetraethylene pentamine，and 2% amount of benzoyl peroxide of total materials quality. Compared with EG-01 and EG-02，the dehydration rate of NG-01 and NG-02 was faster，and the dehydration effect was better. The dehydration effect of NG-02 with triblock structure was better than that of NG-01 with diblock structure. The dehydration rate of NG-02 on heavy crude oil in Shengli oilfield was more than 90%，and that on medium crude oil in Sudan oilfield and heavy crude oil in Lvda oilfield of CNOOC was more than 95%.