An emulsion sizing agent used for carbon fibers with a small contact angle and a low moisture regain is disclosed. The emulsion sizing agent comprises main agents and deionized water. The mass ratio of the main agents to the deionized water is (35:65)-(55:45). The main agents comprise 10-30 wt% of aromatic polyester polyol, 60-80 wt% of bisphenol A epoxy resin, 6-10 wt% of an anionic surfactant, 0.5-1.5 wt% of an antioxidant and 0.5-1.5 wt% of a leveling agent. The carbon fibers sized by utilizing the emulsion sizing agent are small in contact angles with substrate resin, high in infiltration speed, high in adsorbability, high in interface bonding strength with composite materials, extremely low in moisture regain, low in requirements on carbon fiber storage environments, capable of being stored for long time without influences on physical performance of the fibers and high in resistance to heat and humidity, and can be applied in a plurality of complex environment conditions, such as aviation and offshore operation. A preparing method and a using method of the emulsion sizing agent used for the carbon fibers are also disclosed.