In order to develop a process for the synthesis of unrefined tri-n-butyl citrate(TBC) without purification but directly on acetyl tri-n-butyl citrate(ATBC), the high activity and high selectivity of methyl sulfonic acid mixed catalyst was selected to be applied to the esterification reaction firstly, and then proposed further improvement and optimization of the synthesis of TBC. The influences of esterification temperature and distillation conditions on the quality of synthesis TBC were discussed. The results showed that when the ratio of citric acid and butanol was 4.5∶1, amount of catalyst was 1.5 % by mass of citric acid, the esterification temperature was 100～130 ℃ and reaction time was 5.0 h, TBC was synthesized, and was distillated for 20 min at 125 ℃ and dealcoholization, the yield and purity of TBC were above 99%, and TBC could be directly used in the synthesis of ATBC.