The invention discloses a protective agent for quantitatively preserving microbial strains, which comprises 0.1 to 10 mass parts of water-soluble sugar, 0.1 to 5 mass parts of defatted milk powder, 0.1 to 20 mass parts of gelatin and 0 to 10 mass parts of active carbon. The protective agent well protects various microbial strains; after the protective agent is mixed with the strains and subjectedto freeze-drying, the activity of the strains can be ensured when the strains are preserved at 4-8 DEG C in a dark place; moreover, the biochemical characteristics of microbes cannot be influenced, low-temperature freezing is not required and the operation is convenient. Microbial strain freeze-drying products protected by the protective agent can stably survive at the temperature of 20 DEG C formore than 30 days, the requirement on an external environment is not high, and operation of inoculation and the like is greatly facilitated.