A modified preparation technology for a light GMT sheet material comprises: employing base-free glass fiber and low-melting-point modified polypropylene according to a certain weight ratio as raw materials, employing vacuum infiltration technology, placing base-free glass fiber at the bottom of a mold, then paving polypropylene thick resin blocks on glass fiber, sealing with a vacuum bag, in hot environment, employing vacuum technology to suck resin from top to bottom, wherein the mold comprises an injection system, an exhaust system and a heating system, and is carved with grooves for accelerating resin flow, guaranteeing resin smooth flow, exhausting all gas in the mold cavity and thoroughly infiltrating fiber. The preparation technology is high in efficiency, low in equipment cost and small in energy consumption; the whole preparation technology is free of pollution, small in glass fiber damage and high in raw material utilization rate; and the preparation process is simple.