The invention relates to a modified sulfonated melamine-formaldehyde condensation polymer water reducing agent and a preparation method thereof. The obtained modified sulfonated melamine-formaldehyde condensation polymer water reducing agent has the advantages of high dispersing performance, high water reducing rate, small slump loss and high storage stability. The preparation method of the modified sulfonated melamine-formaldehyde condensation polymer water reducing agent comprises the following steps of: (1) undergoing a modification reaction: undergoing a modification reaction on melamine,a first part of formaldehyde and a modifier, wherein the feeding molar ratio of the melamine to the formaldehyde to the modifier is 1:(3.6-5.5):(1.0-1.8); (2) undergoing a condensation reaction, wherein the weight average molecular weight of a condensate is 50,000-80,000; and (3) undergoing a sulfonation reaction: adjusting the pH value to 8-9, adding a second part of formaldehyde and a sulfite sulfonating agent, adjusting the reaction pH value and undergoing the sulfonation reaction, wherein the molar ratio of the melamine to the second part of formaldehyde to SO3<2-> provided by the sulfitesulfonating agent is 1:(0.4-2.0):(0.2-1.5).