The invention discloses a sizing agent for carbon fibers, a preparation method and an application. The sizing agent is in an emulsion form and contains a main sizing agent and a surfactant, wherein the main size is prepared from thermoplastic polyarylene sulfide polymers and water, the thermoplastic polyarylene sulfide polymers account for 0.05wt%-10wt% of the main sizing agent, and the surfactant accounts for 0.05wt%-3wt% of the main sizing agent. According to the sizing agent, mechanical properties of a polyarylene sulfide polymer based carbon fiber reinforced composite can be improved.