The invention discloses a water-viscosity building block toy material composition and a preparation method thereof, the water-viscosity building block toy material composition comprises the followingcomponents in parts by weight: 30-65 parts of polyvinyl alcohol, 1-4 parts of vinyl alcohol copolymer, 1 to 5 parts of a plasticizer, 30 to 60 parts of a filling agent, 0.2-0.5 part of an antioxidant,1-2 parts of a lubricant and 0-6.0 parts of a pigment. The building block toy material composition easy in injection molding is obtained by adopting the polyvinyl alcohol with alcoholysis degree of 88% to compound with the ethylene- vinyl alcohol copolymer, supplementing the plasticizer and the lubricant, and adding the filling material with a proper proportion to obtain the water-viscosity building block toy material composition. The water-viscosity building block toy material composition is simple in preparation process, convenient to operate and control, stable in quality, high in production efficiency, low in manufacturing cost and capable of realizing large-scale industrial production. The water-viscosity building block toy material composition is capable of forming a movable combination body only by coating a connecting surface with water without use of convex particles and grooves for plugging and use of an adhesive for bonding.