Cortisone acetate is a steroidal drug and an important raw material for the synthesis of steroidal drugs such as prednisone. The preparation of cortisone acetate can be carried out in two steps: iodization and substitution reaction. Iodination reaction is the key step. The maximum molar yield of cortisone acetate was 72.74% in the literature. In this work, the iodization reaction at C-21 position by using 21-deoxycortisone (SSC) as starting raw material and bromine iodine solution as iodinating agent was explored. The conversion of SSC to monoiodide and the exchange between monoiodide and diiodide were studied. The effects of molar ratio of SSC to I2 (bromine iodine solution), feeding rate of bromine iodine solution, extended reaction time and reaction temperature on iodination reaction were investigated. The process was optimized in order to increase the yield of monoiodide. The changes of monoiodide, diiodide and SSC were detected by HPLC. The results showed that there were competition between the conversion of SSC to monoiodide and the continued conversion of monoiodide to diiodide. When the contents of monoiodide and iodinating agent reached certain values, following continuous addition of iodinating agent, the conversion of monoiodide to diiodide became the main reaction, which led to the increase of SSC in the reaction solution. It was also observed that diiodide could be converted to monoiodide during extended reaction process, therefore the yield of monoiodide could be improved. All the studied factors had significant effects on the iodination reaction. The optimized conditions for the synthesis of monoiodide were as follows: molar ratio of SSC to I2 (bromine iodine solution) 1:0.62, feeding rate of bromine iodine solution 0.33 mL/min (SSC 20 g), extended reaction time 120 min, reaction temperature ?3℃. Under these optimal conditions, the molar yield of cortisone acetate reached 87.67%, which was 14.93% higher than the yield of literature reported. This optimized process could provide better economic benefits.