The invention discloses a composite drill bit used for breaking rocks in a ploughing and cutting combined and alternating mode. The composite drill bit mainly comprises a drill bit body, a fixed cutting structure provided with cutting teeth, and at least one rotating gear rotatably connected with the composite drill bit. Ploughing teeth are arranged on each rotating gear capable of rotating relative to the drill bit body. The rotating gears have certain axial deviation relative to the center of the drill bit body, so that the ploughing teeth on the rotating gears can be used for breaking the rocks alternately. According to the rock breaking mode, micro impact enters and squeezes the rocks. The rocks are crushed due to a failure of tensile stress. The ploughing rock breaking mode through the ploughing teeth and the shearing rock breaking mode through the cutting teeth on the fixed cutting structure are combined for breaking the rocks, accordingly the working stability of the drill bit is improved, the service life of the whole drill bit is prolonged, and the rock breaking efficiency of the drill bit is improved.