This paper explored ozonation for the treatment of pharmaceutical wastewater and p-toluenesulfonic acid (PTSA) was selected as the model pharmaceutical intermediate. The results showed that the ozonation process is efficient for the degradation of PTSA both in concentration and COD removal. The mono factor results showed that 60L/h was a suitable ozone flow rate. There were sharp difference between acid condition and alkaline condition, under acidic conditions, the direct oxidation with molecular ozone is of primary importance, and under alkaline conditions the hydroxyl oxidation starts to dominate, and pH=12 was a suitable value. Further, intermediates of PTSA degradation were analyzed and the reaction mechanism was discussed.