The invention discloses a sheet molding compound free of magnesium oxide thickening and a preparation method thereof. Application of a magnesium oxide thickening agent is avoided, polyurethane modified vinyl resin is added and used as matrix resin, and in cooperation with other components including reinforcement fibers, a low-shrink additive, inorganic mineral filler, a curing agent and a mould release agent, by selecting the optimal preparation method and technological parameters, the sheet molding compound is prepared; the problem of unstable thickening of the magnesium oxide powder thickening agent during use is solved, polyurethane modified vinyl resin can bear higher impact force compared with common unsaturated resin and has good creep resistance and fatigue resistance, the product quality and performance of the sheet molding compound are improved, the sheet molding compound can be more widely applied to the fields such as manufacturing of industrial pump valves, automobile battery boxes and seats, and the application range is wider.