Acetylsalicylic acid is a widely used non-steroidal anti-inflammatory drugs. A method of visible spectrophotometric was developed for the determination of salicylic acid and hydrolysis kinetics of acetylsalicylic acid based on the fact that salicylic acid and ferric complex with voilet color. In the buffer solution of HCOOH-NaOH with pH of 3.0, a type of 1∶1 stable complex of salicylic acid and ferric was formed and apparent stability constant (lg K) was calculated to be 6.02. Absorption maximum of the complex was found at the wavelength of 523 nm. Linear relationship was kept in the range 0.05-0.5 mmol/L and detection limit(3 s/k) was 2.2 μmol/L. The recovery rates measured were in the range of 96.0%-100.5%, and RSD were in the range of 0.76%-1.10% when the methods was applied to sample. Results of kinetics equation fitting with online monitoring data show that acetylsalicylic acid hydrolysis accorded with first order reaction and activation energy was 72.97 kJ/mol. It would be helpful to study and determine parameters of chemical kinetics.