Taking a graphite ore in Luobei, Heilongjiang Province as the research object, in order to improve the fixed carbon content of graphite flotation concentrate, through pure mineral flotation test, the mechanism of typical gangue quartz entering graphite concentrate was analyzed, and the effects of ultrasonic time and ultrasonic power on the flotation of graphite and quartz artificial mixed ore were explored. The actual ore flotation test was carried out by ultrasonic assisted process to evaluate the effect of ultrasonic pretreatment on the fixed carbon content of graphite concentrate. The results of pure mineral flotation tests show that graphite forms flocs under hydrophobic interaction, causing fine-grained quartz to enter the concentrate through entrainment and inclusion. Ultrasonic pretreatment can strengthen particle dispersion, reduce water recovery and reduce quartz floating. The actual ore flotation test show that the graphite concentrate with fixed carbon content of 94.61% and recovery rate of 81.66% is obtained by using the open-circuit flotation process of "five grindings and six flotations" and ultrasonic pretreatment for 3 min before the third re-election to the sixth re-election flotations. The fixed carbon content of the concentrate is 3.10 percentage points higher than that without ultrasonic treatment. The research results provide a reference for improving the fixed carbon content of graphite concentrate in the re-grinding and re-flotation stage by ultrasonic assisted process.