The invention relates to a preparation method of a fixing agent for composite papermaking and belongs to the technical field of pulping and papermaking. The preparation method includes: subjecting duck's down to steps of surface treatment, dissolution and acid precipitation to obtain dried powder; using potassium nitrate and anhydrous magnesium chloride as raw materials for mixing to obtain modifying liquid; mixing perlite powder with the modifying liquid, soaking, filtering, drying filtering residue, and calcining to obtain modified expanded perlite; mixing juice obtained by juicing aloe with the modified expanded perlite and the dried powder, heating, adding a cationic etherifying agent solution for reaction, evaporating a reaction product to be dry, and smashing to obtain the fixing agent. The fixing agent is less prone to enabling size to excessively flocculate, thereby improving uniformity and strength of paper, and has wide application prospect.