Salt content of output crude oil in Missan oilfield of Iraq is often higher than 28 mg/L stipulated in the contract，which affects the production increasing plan，and therefore the demulsifier is urgent to be optimized. Based on the crude oil treatment process in Missan oilfield，the effect of dehydration temperature and emulsification times on demulsification capability was investigated，and the evaluation conditions were determined. The optimum formula HYP-243 was developed by bottle test and electric dehydration method. Finally HYP-243 was applied in Missan oilfield. The experiment results showed that when the dosage of demulsifier was 80 mg/L，the temperature was 60℃ and the emulsification times was 150，the demulsifier HYP-243 formulated by alkylphenol formaldehyde resin demulsifer YFPC-541，phenol-amine resin demulsifier YFPG-242 and aliphatic amine demulsifier YFPC-140 in mass ratio 4∶1∶0.5，had better dehydration performance than demulsifier RP-968，such as faster dehydration rate，higher final dehydration volume and cleaner water quality. The field test results showed that better salt leaching effect was achieved when the dosage of HYP-243 was 75 mg/L. The average salt content in outer wash tank site decreased from 573 mg/L to 358 mg/L，and that in output crude oil decreased from 36 mg/L to 22 mg/L，even if the dosage was reduced by 10 mg/L. The results showed that HYP-243 could meet the requirements that the salt content of output crude was lower than 28 mg/L，which was helpful for the high and stable production.