The synthesis of the C.I. Pigment Red 177(PR177) was developed by the desulfonation of 4,4'-diamino-1,1'-dianthraquinonyl-3,3'-disulfonic acids(DAS) in ionic liquids ([BSMIm]HSO4, [BSMIm]OTF) instead of H2SO4. The influences of the acidity, dosage and recycle of ionic liquids, reaction time, reaction temperature on the yield of PR177 were studied. The acidity of ionic liquids satisfied the acidity requirement of desulfonation of the DAS. The recycle of ionic liquids was convenient, and it reduced the discharge of wastewater. The synthesis technology was environmentally friendly. The optimal reaction conditions were as follows: The dosage of DAS and [BSMIm]OTF were 2.0g and 10g separately, the reaction temperature was 150℃, the reaction time was 5h, the number of recycle of ionic liquids after dichloromethane extraction was 6, the average yield of PR177 was 92.3%.