A modified belousov–zhabotinskii (B–Z) oscillating–chemical reaction with I – ion is used to detect the rhodamine B in capsicum powder. The results show that detection sensitivity for rhodamine B is obvious improved in the modified detection system. When the concentration of malonic acid is 0.105 mol/L, the concentration of potassium bromate is 5.8010–2 mol/L, the concentration of cerous sulfate is 1.6010–3mol/L, the concentration of kalium jodatum is 2.5×10–4 mol/L, and the concentration of sulphuric acid is 0.7mol/L, rhodamine B showed the greatest perturbation on the modified system. When concentration of rhodamine B is 7.5´10–7 mol/L to 7.0 ´10–4 mol/L, the changes of amplitude is linearly proportional to the –lgC, where C is the concentration of rhodamine B. The detection limit is 8.710–8 mol/L and the respective regression coefficient is 0.998 7.