At present, tetra-hydroxymethyl phosphonium chloride (THPC) is widely used in flame retardant finishing, industrial water treatment and leather manufacture industry etc., and its decomposition will affect the actual application. So the thermal decomposition and acid-alkali decomposition of THPC were studied by 31P nuclear magnetism resomance (31P NMR), thermo-grav imetric analysis (TGA) and differential scanning calorimetry (DSC) respectively. The results showed that THPC solution was stable when pH<5.0, containing THPC, tri-hydroxy methyl phosphine (TrHP), and tri-hydroxy methyl phosphine oxide (TrHPO ). THPC began to decompose at pH5.0 and yielded an unstable substance tetra-hydroxy methyl phosphonium hydroxide (THPH ), whose chemical shifts was 36ppm. At pH8.0, THPC converted to TrHP and TrHPO completely. When pH>9.0, all of the phosphorus compounds converted to TrHPO. Consequently, THPC content decreased when pH of the THPC solution rose. Thermal decomposition experiment was also carried out. The structure of THPC began to change when heated to 152.4°C and lose weight at 184.41°C. Therefore, application temperature should be below 152°C, which could give a favorable guide in THPC application.