The invention relates to a preparation method of indoor flatting powder paint, which is characterized by comprising the following steps: adding polybasic acid or acid anhydride, polyalcohol, catalyst and antioxidant into a reaction kettle, mixing, introducing nitrogen, heating to carry out esterification reaction or ester exchange reaction until the reaction liquid is completely clear, carrying out vacuum polycondensation until the acid value is 40-200mg KOH/g and the viscosity is 10-50 poise, cooling, and discharging to obtain the light yellow transparent or semitransparent solid component A polyester resin; adding polybasic acid or acid anhydride, polyalcohol, catalyst and antioxidant into a reaction kettle, mixing, introducing nitrogen, heating to carry out esterification reaction or ester exchange reaction until the reaction liquid is completely clear, carrying out vacuum polycondensation until the acid value is 10-50mg KOH/g and the viscosity is 30-100 poise, cooling, and discharging to obtain the light yellow transparent or semitransparent solid component B polyester resin, wherein the molecular weight of the component A polyester resin and the component B polyester resin are respectively controlled at 1000-10000 dalton; evenly mixing the component A polyester resin, component B polyester resin and polybasic organic acid to obtain a component C polyester resin; and mixing the component C polyester resin, epoxy resin E-12, barium sulfate, titanium white, leveling agent, brightener, physical flatting agent and benzoin to obtain the indoor flatting powder paint. The synthesis method has the advantages of diversity and the like, and can easily implement industrialized mass production.