1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) has excellent stability and very low mechanical sensitivity and is used as an important component in polymer binder explosive (PBX) for nuclear weapons. However, at present, the methods of industrial preparation of TATB have problems such as low efficiency and residual impurities. Based on a new method to synthesize TATB from phloroglucinol, we developed a continuous and efficient method for preparing 1,3,5-triethoxy-2,4,6- trinitrobenzene (TETNB), the key intermediate to synthesize TATB. In view of the importance of this reaction, in-depth theoretical research on the reaction mechanism has been conducted with the Gaussian 16 software at the M062X/6-31+G** level. The reaction mechanism and the thermodynamic equilibrium of the etherification process were clarified. Under the catalysis of protic acid, triethyl orthoformate eliminates one ethanol molecule to form a diethoxy-carbocation intermediate. Since the lone pair electrons of the adjacent oxygen atoms can partially fill the empty p orbital of the carbocation, the diethoxy-carbocation intermediate is relatively stable. Subsequently, there are two reaction modes for diethoxy-carbocation. In the first mode, trinitrophloroglucinol (TNPG) anion acts as a nucleophile to attack the diethoxy-carbocation intermediate in S_N2 fashion, grabbing an ethyl carbocation and achieving the alkylation. In this mode, the diethoxy-carbocation intermediate directly provides an alkyl carbocation. In the second mode, addition reaction occurs between the TNPG negative ions and the diethoxy-carbocation intermediate to produce new orthoformate, which can be regarded as transesterification. Then, the ethanol molecule conducts an aromatic nucleophilic substitution (S_NAr) to replace the diethyl orthoformate on the aromatic ring and complete the alkylation. In comparison, diethyl orthoformate is a relatively better leaving group. In addition, this etherification reaction has a remarkable feature: the thermodynamic stability of the product is slightly worse than that of the reactants. Therefore the yield is lower when chemical equilibrium is reached. This requires continuous removal of the by-products of the system in the experiment to pull the chemical balance toward the direction of the product.