The invention discloses a garbage dump anti-seepage pulp material based on sodium carboxymethyl cellulose modification and a preparation method thereof. The anti-seepage pulp material is composed of cement, bentonite, fly ash, sodium carboxymethyl cellulose, aluminum salt polycarboxylic acid high-performance water reducing agent and sodium carbonate, wherein sodium carboxymethyl cellulose, sodiumcarbonate and aluminum salt are used as modifier for bentonite. The hardening rate of the anti-seepage pulp material is greater than 99.9%, and the consolidated body 28d permeability coefficient is 0.55*10<-8>-6.7*10<-10> cm/s, unconfined compressive strength is 0.40-1.95 MPa; the blocking rate of the slurry consolidated body on CODCr, BOD5, NH4-N, TP and SS is over 96.5%, and the blocking rate onthe heavy metal ions such as Hg and Pb is over 99.95%. The slurry consolidated body has good durable stability, suitable compressive strength and elastic modulus, good plasticity and toughness, and coordinates with the foundation deformation of the landfill site, so as to ensure that the impermeable separation wall does not crack and fail during the design service life. The anti-seepage slurry material of the invention has the advantages of low cost, non-toxic and environmental protection, strong controllability of setting time, and convenient preparation and use on the construction site.