The invention discloses a plant-based active flame-retardant fire extinguishant, and a preparation method and an application thereof. The plant-based active flame-retardant fire extinguishant is prepared through mainly compounding 15-20% of a plant-based anti-burning agent, 8-17% of a plant-based composite flame-retardant activator and 63-82% of plant residue ash water; and the fire extinguishantis used after being mixed with water to achieve a concentration of 1-5% and added into a fire extinguishing device. Various plant fruits are added, so vegetable proteins having different sources, pectin, tannin-rich polyhydroxy compounds, lignin and the anti-burning substance are increased, thereby the plant-based active flame-retardant fire extinguishant has a strong fire extinguishing function and an ideal fire extinguishing effect inaccessible to existing fire extinguishants; and the plant-based active flame-retardant fire extinguishant has the advantages of easily available raw materials,low price, simple preparation process, simple device conditions, low cost, convenience in use, and suitableness for attacking class A, B and C large-area fires.