Cotton and its blended fabrics are widely used textile materials in daily life; however, their high flammability brings serious fire hazards. It remains a huge challenge to endow cotton and its blended fabrics through simple and universal technology. In this work, a novel water-based polyelectrolyte complex (PEC) coating was developed from chitosan and a water treatment agent (amino trimethylphosphonic acid). By controlling the degree of ion pair aggregation through pH adjustment, the intumescent and stable PEC coating can be constructed quickly via facile blading coating and curing processes. High transparency allows for the deposition of the coating with little effect on the original pattern of the coated fabric. Additionally, when the coated fabrics were exposed to flame, the PEC coating underwent a quick self-char-forming reaction in advance and formed a protective char layer covered on the fibers. Thus, the PEC coating with low adding-on (11.5–21.5 wt %) shows universally high flame retardancy for cotton, polyester/cotton blends, and polyester/poly(vinyl alcohol)/cotton blends. The coated fabrics show excellent extinguishing performances, high limiting oxygen index values of 27.5%–28.5%, and total release smoke inhibition of 42%. This work provides a promising universal technology for manufacturing fire-safety cotton and its blends in a simple and eco-friendly way.