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 which was caused by pH changing will influence on the actual application. So acid-alkali decomposition of THPC was studied by 31P Nuclear Magnetism Resomance ( 31P NMR), spectrophotometry and titration respectively. The results show that THPC solution is stable when pH<5.0, containing THPC, tri-hydroxymethyl phosphine (TrHP), and tri-hydroxymethyl phosphine oxide (TrHPO). THPC begins to decompose at pH5.0 and yields an unstable substance tetra-hydroxymethyl phosphonium hydroxide (THPH), whose chemical shifts is 36ppm. At pH8.0, THPC converts to TrHP and TrHPO completely. When pH>9.0, all of the phosphorus compounds converts to TrHPO. Consequently, THPC content decreases when pH of the THPC solution rising, which could give a favorable guide in THPC application.