The invention discloses a method for producing a terephthalic acid through the oxidation of p-xylene by using air based on a gas-liquid-solid heterogeneous reaction separation synchronizing reactor, which comprises the following steps that: p-xylene is added into the gas-liquid-solid heterogeneous reaction separation synchronizing reactor, wherein the reactor comprises a reaction tower and at least two constant-temperature subsidence towers communicated with the bottom of the reaction tower, a gaseous phase area, an oxidation area and a gas outlet are arranged in the reaction tower; in the process of reaction, the oxidation area and the constant-temperature subsidence tower are filled with p-xylene, air is continuously fed from bottom of the oxidation area so as to contact and react with the p-xylene, and generated terephthalic acid solids enter the constant-temperature subsidence tower by settling, and meanwhile, the p-xylene in the constant-temperature subsidence tower enters the oxidation area through an outer circulatory system, so that the reaction is running continuously, the p-xylene continues to be settled in the constant-temperature subsidence tower until the constant-temperature subsidence tower is filled, then the reaction tower is switched to be communicated with the other constant-temperature subsidence tower filled with p-xylene, in such an alternate way, continuous production is achieved. According to the method, a terephthalic acid can be continuously produced at high conversion rate and high yield, so that the production efficiency is improved, and the production cost is reduced.