To discover the change pattern of L-Cysteine(L-Cys) in food processing,the optical rotation of L-Cys and L-Cystine(L-Cys-Cys) solution were determined under different pH and environment(H2O2 and NaHSO3).The results showed that,at room temperature and pH=3.0~8.0,the oxidation reaction of L-Cys followed zero order reaction and the rate constant was the smallest at pH 3.0,and the biggest at pH 4.0 and 8.0.Whether the solution is aerobic or not influenced the initial concentration of L-Cys,but would not have impact on its rate constant.H2O2 and NaHSO3,which are used as pro-oxygenic agent in L-Cys solution,could increase the oxidation reaction rate constant,to 9×10-6 mol/(dm3·h) and 2×10-6 mol/(dm3·h),respectively.So NaHSO3 could not be antioxidant for protecting L-Cys.L-Cys was thermal instability when heated above 60 ℃.