A crude demulsifier for gathering and dewatering crude in oil field features that glycol or propanediol as initiator and the mixture of alkali-earth-metal compound and alkali-metal hydroxide as catalyst are used to initiate the copolymerization of epoxy ethane or epoxy propane to obtain linear polyether, which reacts with polyisocyanate in arene solvent to obtain said demulsifier. Its advantages are high demulsifying and dewatering effects, and wide adaptability (especially for viscous crude).