This invention discloses a polyester resin for indoor powder paint synthesized from crude trimellitic anhydride (with purity of 90-95%) and waste polyester material alcoholysate and a preparation method thereof. The method comprises the steps of: taking waste polyester as the raw material, adopting 1, 3-diethylimidazole fluoroborate as the catalyst, selecting the catalyst content and the process temperature to regulate the molecular weight and distribution of the obtained diol compound, and when the molecular weight and distribution are within an appropriate range, replenishing terephthalic acid, isophthalic acid and the catalyst according to the formula to complete stepwise polymerization reaction, and performing vacuum pumping to carrying out condensation polymerization reaction, and using crude trimellitic anhydride (with purity of 90-95%) to block the end, thus finally obtaining the polyester resin for indoor powder paint. The resin has low melt viscosity, high glass transition temperature, and narrow molecular weight distribution, and the powder paint prepared from the resin has leveling performance and luster superior to the prior art.