The N-(2- Hydroxyl ethyl)-3- methyl imidazole terafluoroborate ([C2OHmim]BF4) were synthesized under the conditions of ultrasound intermittent operation．The yield was 57.69％. Compared with conventional synthesis, the reaction time was drastically reduced and the higher yield was obtained． Then [C2OHmim]BF4 was used for aqueous two-phase system (ATPS). The result shows that salt kind and its concentration had important influence on ATPS. The thermal stable ATPS can be formed as follows: 2mL [C2OHmim]BF4 , 3mL water , 0.7500g (NH4)2SO4 and 25 degree. The phase volume ratio of system was 2.4∶2.6 about PH 6～9. [C2OHmim]BF4can be reused for extraction.