The invention provides a preparation method of a mud-resistant modified-naphthalene water reducing agent. The mud-resistant modified-naphthalene water reducing agent related to by the invention is prepared by the following steps: firstly preparing a cationic monomer, carrying out a polycondensation reaction on formaldehyde with a product obtained by sulfonation of concentrated sulfuric acid, afterthe reaction is carried out for a certain time, adding the cationic monomer for continuous reaction for a certain period of time, so as to obtain the mud-resistant modified-naphthalene water reducingagent. The mud-resistant modified-naphthalene water reducing agent provided by the invention is a novel water reducing agent containing zwitterions, and not only has a high-efficiency water reductioneffect, but also has a strong mud-resistant property.