The invention discloses a low-cost self-protection flux-cored wire containing a multi-granularity deoxidizing agent and a preparation method of the low-cost self-protection flux-cored wire containing the multi-granularity deoxidizing agent. A mild-carbon steel strip serves as a skin, and a flux core is prepared from 62 wt% to 75 wt% of high carbon ferro-chrome, 10% to 25% of the deoxidizing agent and the balance iron powder. The deoxidizing agent is a mechanical mixture formed by graphite, cellulose powder, an aluminum-magnesium alloy and a silicon-manganese alloy, the graphite, the cellulose powder, the aluminum-magnesium alloy and the silicon-manganese alloy are each composed of at least two different granularities, and the flux core accounts for 52 wt% to 56 wt% of the mass of the flux-cored wire. According to the flux-cored wire, deoxidization is full in the whole welding stage, the welding processing property is remarkable, a bead weld layer has the beneficial effects of being high in hardness, good in wear resistance and the like, the surface of a weld bead is basically free of slag, slag removal is not needed during multilayer welding, and the manufacturing cost of the flux-cored wire is low.