The invention relates to an environmental-friendly water-based fire extinguishing agent, which comprises: deionized water, an antiseptic agent, a metal antirust agent, a halogen-free fire retardant, a hydrocarbon surfactant, a high efficiency chelating agent, 2-5 parts of an osmotic agent and a high efficiency corrosion inhibitor. The preparation method of the water-based fire extinguishing agentconsists of the steps of: adding deionized water into a dispersing cylinder according to the weight part, then adding the antiseptic agent, the metal antirust agent, the halogen-free fire retardant and the hydrocarbon surfactant, and stirring the mixture uniformly till completely salvation of the mixture; then adding the high efficiency chelating agent, the osmotic agent and the high efficiency corrosion inhibitor, stirring them uniformly till semi-clarity, thus obtaining the finished product. Containing a halogen free fire retardant, the fire extinguishing agent of the invention can be biologically degraded 100%, thus having no pollution to surrounding equipment and space. Containing an osmotic agent, the fire extinguishing agent in the invention can penetrate articles in fire hazards ofclass A, and can penetrate into the inside of combustible articles, so that even if the fire is not completely put out due to a large fire, the part injected with the agent can effectively block a fire source and control the spreading speed of a fire. Therefore, the fire extinguishing agent provided in the invention has excellent flame resistance.