The invention provides a composite flocculant and a preparation method thereof, and an oil field sewage treatment method. The preparation method of the composite flocculant comprises the steps: mixing polyaluminium chloride and an Al(OH)3 colloid to obtain a first mixture; and carrying out primary curing of the first mixture to obtain the composite flocculant. The Al(OH)3 colloid and the polyaluminium chloride are subjected to mixed curing, so that the adsorption effect of the polyaluminium chloride on solid suspended particles and an effect of bridging to form flocs are in mutual synergistic promotion with the carrier adsorption effect of the Al(OH)3 colloid, then the composite flocculant not only provides mutual contact carriers for the solid suspended particles due to the presence of the Al(OH)3 colloid and promotes the formation of the flocs when in use, but also enhances the bridging effect, improves the floc morphology, and accelerates the floc settling speed, so as to improve the ability of sewage purification.