The hydrolysis of polyphenyl-1,2,4-triazine (As-PPT) and polyphenyl-1,3,5-triazine (S-PPT) in water at 250℃/ 3.97 MPa under nitrogen atmosphere has been investigated experimentally and theoretically. The hydrolysis reactions were monitored by Fourier transform-infrared (FT-IR) and ultra-violet-visible (UV-Vis) spectra. The results show that S-PPT is remarkably stable whereas As-PPT is easily hydrolyzed in water at 250℃. The identifications using FT-IR, high performance liquid chromatography (HPLC), nuclear magnetic resonance (1H-NMR) and mass spectrography (MS) confirm that terephthalic acid is the major hydrolysate of As-PPT. Two model compounds, 2,5,6-triphenyltriazine and 2,4,6-triphenyltriazine, were designed to simulate the local electronic structures of As-PPT and S-PPT, respectively. The electronic structures were given by ab initio calculations at the RHF/4-31G level. The calculational results indicate that the triazine rings are the hydrolytically active parts and the 1,2,4-triazine ring is easier to hydrolyze in comparison with the 1,3,5-triazine ring under the same condition.