For swimming, mosquito larva (Chironomus plumosus) bends its cylindrical body into nearly a circle from one side to the other sequentially and generates a trajectory that resembles figure-of-eight. This gait is drastically different from the undulatory gait used by other slender-body organisms, such as eel and nematode. We developed a numerical model of the larva with prescribed body deformation and coupled it with computational fluid dynamics. We found that, in contrast to undulatory swimmers who try to reduce rotation all the time to align their propulsion with the direction of motion, the mosquito larva purposely generates a significant rotation and timely generate the propulsion forces when the propulsion forces align with the direction of motion. The mosquito larva also modulates the speed of body deformation to increase the propulsion force since force scales more than linear to speed at intermediate Reynolds numbers. As a result, the swimming speed is greater than the speed of the undulatory gait in the same condition. Although the energetic efficiency of the figure-of-eight gait is low, the power is small compared to the larva’s metabolic rate.