The invention relates to a method for preparing a sizing agent for sizing of poplin type tightly-woven fabric fiber. According to the method, polyoxyethylene, polyoxypropylene ether, polyacrylamide, di-rhamnolipid and sizing paste are added to a flask containing a stirrer, a thermometer and a reflux condensing tube, deionized water and taurine are added, temperature is increased to 85 DEG C, nitrogen is introduced for protection, stirring is conducted for 40-50 min, citric acid is slowly added for about 2 h, heat preservation is conducted for 2 h, then a PMA sizing agent, laurinol polyoxyethylene ether and ethanol are added for reaction for 9-12 h, temperature is lowered to indoor temperature, and then the sizing agent for sizing of the poplin type tightly-woven fabric is obtained. The sizing agent prepared with the method has high fiber adhesive force and high cohesive force after sizing is conducted, and fiber fuzzing can be prevented through a surface film formed on the surface of the fiber; furthermore, decoating or reduction of fiber strength caused by friction is avoided, fiber strength can be improved, and meanwhile, the elasticity, flexibility and extensibility of the fiber are maintained.