The invention discloses a composite coal water slurry additive and a preparation method thereof. The composite coal water slurry additive is mainly prepared by neutralizing a naphthalene sulfonate formaldehyde condensation product intermediate and a sodium carboxymethyl cellulose intermediate. The preparation method mainly comprises the steps of preparing the naphthalene sulfonate formaldehyde condensation product intermediate, preparing the sodium carboxymethyl cellulose intermediate, carrying out the neutralization reaction and the like. Since phosphate is added in during the neutralization reaction, the viscosity-reduction property of phosphate is utilized to improve the technique, consumption of chemical materials is reduced, the content of inorganic salt in the composite coal water slurry additive is reduced, and the use performance of the product is improved; the composite coal water slurry additive is mainly used for preparing coal water slurry, so that coal water slurry has lower viscosity and better fluidity in normal use.