The present invention relates to the field of petroleum product processing, particularly to a method for synthesizing nonylphenol from nonene and phenol through catalysis with an alkaline liquid. The method comprises: adding phenol and an alkaline ionic liquid catalyst to a three-necked flask, carrying out stirring heating to a temperature of 70-100 DEG C, and adding nonene in a dropwise manner, wherein the adding time is 2-6 h; continuously carrying out a constant temperature stirring reaction for 1-3 h after completing the adding; cooling to a temperature lower than room temperature, solidifying the alkaline ionic liquid, and finally separating the product and the alkaline ionic liquid to prepare a nonylphenol mixture; and pre-separating the excess phenol from the mixture, and rectifying to obtain the nonylphenol. Compared to the traditional preparation method, the preparation method of the present invention has advantages of environmental protection, no corrosion to equipment, low catalyst consumption, rapid reaction, high product quality, easy separation of the product and the ionic liquid catalyst, low reaction temperature, and the like.