Methylnaphthalene sulfonic formaldehyde condensate (MNSF) was synthesized from industrial methylnaphthalene by four steps: firstly sulfonated by H2SO4, then hydrolysised by H2O, nextly condensated by HCHO and neutralized by NaOH. The factors influencing the dispersion ability of MNSF used as superplastiser were investigated. The optimum reaction conditions were described as below: the mole ratio of material is n(methylnaphthalene): n(H2SO4): n(H2O of hydrolysis): n(HCHO):n(H2O of condensation) =1:1.25:1.25~1.5:0.92:4.6; sulfonation temperature was 160～165 ℃; sulfonation time was 3h, hydrolysis temperature was 110～120℃; hydrolysis time was 15～30min; condensation temperature was 110℃; condensation time was 4 h; the acidity value before and after hydrolysis was controlled near 30%. When the dosage of MNSF was 0.5%, the water-reducing ratio reached to 16% and 4% more than that of FDN, the flexural and compression strength of added MNAF cement mortar approached that of FDN.