The invention discloses a dispersing agent for decreasing viscosity and a solidifying point of thick oil and a preparation method thereof. The dispersing agent comprises the following components by mass fraction: 10% to 30% of an organic solvent, 50% to 80% of light oil, 2% to 10% of a polyether nonionic surfactant and 5% to 15% of ethylene-vinyl acetate copolymers, and the balance being water. The preparation method is simple, and comprises the steps of adding light oil, the polyether nonionic surfactant and the ethylene-vinyl acetate copolymers into a reaction flask, uniformly stirring the materials, increasing the temperature to 30 DEG C to 50 DEG C, and continuously stirring the materials for 1 h to 2 h; and adding the organic solvent, uniformly mixing the materials, and cooling the materials to the room temperature. According to the dispersing agent for decreasing the viscosity and the solidifying point of thick oil, fluidity of thick oil can be effectively improved, thick oil isprevented from condensing, the viscosity and the solidifying point of thick oil are effectively decreased, and transportation is facilitated; and besides, the preparation method of the dispersing agent for decreasing the viscosity and the solidifying point of thick oil is easy and feasible, utilized raw materials are wide in source, cost is relatively low, and popularization and utilization are facilitated.