The invention discloses a quetiapine synthesizing method. O-chlorobenzoic acid with the low price is adopted as a starting material to react with thiophenol, and then ring closure is performed to obtain thioxanthone. Hydroxyl amination and Beckmann rearrangement are performed to obtain a key intermediate dibenzo[b,f][1,4]thiazepines-11-(10H)one, chlorination is performed, then, a reaction is performed on 1-(2-hydroxyethoxy)ethylpiperazine with the existence of acid-binding agent to obtain quetiapine, and the quetiapine and fumaric acid form a salt in an absolute ethyl alcohol system to obtain a product. According to the quetiapine synthesizing method, raw materials are low in price and easy to obtain, the steps are simple, operation is easy, and the cost can be effectively lowered. According to the method, the high-purity quetiapine can be obtained, the liquid phase purity of the obtained semi-fumaric acid quetiapine obtained through salt forming is 99% or above, and the quetiapine synthesizing method can be applied to the field of medicine.