Coal spontaneous combustion and coal fire accidents always lead to a large number of coal resource losses and casualties of workers. Moreover, the environment is polluted and the greenhouse effect is aggravated at the same time. At present, it is difficult for commonly used fire extinguishing materials like water and ordinary foam to suppress the coal spontaneous combustion effectively for a long time and to put out fire quickly. In this paper, a pollution-free fire-retardant foam with high foam expansion and good stability was proposed. The concentrations of SDBS, Guar and MgCl2 were determined as 2%, 0.5% and 1% respectively by the orthogonal test. In addition, the flame retardant property was investigated from three aspects, namely free radical, functional group and CO generating rate. Meanwhile, the fire extinguishing property was investigated through the rate and time of reducing the coal temperature when putting out fire. The results show that the new fire-retardant foam is more effective than water and ordinary foam in inhibiting free radical production and coal‑oxygen composite reaction. Thus, it can inhibit coal spontaneous combustion and the spread of coal fire better. In the low temperature stage, the new fire-retardant foam mainly relies on physical inhibition methods such as water evaporative cooling and isolation of coal and oxygen contact. In the high temperature stage, it mainly relies on chemical inhibition methods such as MgCl2 inhibiting coal‑oxygen complex reaction. Compared with water and ordinary foam, fire-retardant foam can extinguish coal fire and prevent smolder.