The invention relates to an online preparation method of an antistatic polyester cotton yarn of sirofil composite spinning, and belongs to the technical field of textile processing. The method has a characteristic of long filament winding to short fiber by using a method of sirofil composite spinning, a modification device is arranged at the operating path portion from a filament cylinder to a front roller on a ring throstle, and a drying device is arranged at the operating path portion of spun yarn between the front roller and a spun yarn tube. The modification device applies an antistatic agent to polyester filament, the drying device dries the twisted spun yarn, a yarn with an anti-static function is prepared by using a structure characteristic of the polyester yarn in a shape of spatial spiral wrapping on the outer side of cotton roving. According to the online preparation method of the antistatic polyester cotton yarn of sirofil composite spinning, spinning is synchronized with modification, compared with the method of adding modification process after spinning or weaving, the method shortens the production cycle, saves the production cost, and simplifies the operation steps.