Abstract: Ammonium salt of nitrilotriacetic acid was prepared and used as complexing agent for removing calcium of crude oil, using crude of the 2nd distillation unit of Karamay Petrochemical Company as feed. Test results showed that this novel complexing agent exhibited good calcium removal effect under mild operation conditions and with less corrosion to equipment. Using said complexing agent with a mass fraction of 50 μg/g, treating and separating water from oil both at 120 ℃, with a crude flow rate of 2 m/s, the calcium mass fraction of the treated crude oil dropped from 7.1 μg/g to less than 3 μg/g.