The invention relates to a method for preparing a carbon material at low temperature and co-producing a fertilizer. Processes of carbon source pyrolysis and carbon material activation are integrated; through a green process route in a closed system, low-temperature preparation of carbon is achieved; the produced carbon material can be further activated, modified and the like. By the method, the defects of high energy consumption, low yield, high dust pollution, a complicated posttreatment process, and serious environment pollution and difficult treatment of a waste solution and the like in the current carbon producing process are overcome; through the control of the types and the addition amounts of a carbon source and an activator as well as the temperature and the residence time in a reaction kettle, continuous clean production of the carbon material is achieved; through adjustment of the pH or the components, the produced waste solution is used as the fertilizer, so that harmless treatment and high-value conversion of the waste are achieved. In the method, the carbon source is wide (including biomass carbon, tar carbon, bitumen carbon and the like); by the method, the reaction condition is mild, the yield is high, the waste emission is avoided, the energy is saved and the consumption is lowered; therefore, the method is an environmentally friendly, efficient and economical method for sustainably producing the carbon material and co-producing the fertilizer.