The invention belongs to a main raw material for rigid polyurethane foam, and provides flame-retardant composite polyether for on-site spraying. The composite polyether is prepared from following raw materials according to a ratio: L-4110C polyether, triethanolamine, 3152 polyester, tris(1-chloro-2-propyl)phosphate, A33, silicone oil (8805), 1,3,5-tris[3-(dimethylamino)propyl]hexahydro-1,3,5-triazine, potassium oleate, pentamethyldiethylenetriamine, dibutyltin dilaurate and dichlorofluoroethane. Compared with products of the same type, the composite polyether has obvious advantages and is advantageous in that: a polyether cost is low; a formula is simple; during using, products of the composite polyether are milky white, short in wire drawing and debonding time and good in adhesion force; the oxygen index can reach 30; the free bubble density can reach about 40, and flame retardant effects are ensured.