The invention relates to an experiment-grade RTM (Resin Transfer Molding) constant-pressure injection system and a constant-pressure injection method. The system realizes RTM single-component constant-pressure injection through simple and easy connection of a vacuum gauge, a storage tank, a vacuum tank, a vacuum pump, a flash collecting tank, pipelines and valves. The system is simple and convenient to disassemble and assemble and easy to clean, and meanwhile can realize a constant-flow-velocity injection function in an injection process by regulating the injection pressure with an air compressor. The system has high matching capacity, and is simple in structure, low in cost, convenient to disassemble and clean and suitable for forming of experimental samples and small-sized parts of factories and scientific research institutions. The system is used for laboratory RTM research, small-batch production of experimental samples, reduction of the production cost and RTM technology research, and meanwhile can realize vacuum auxiliary injection molding.