The in situ spectroelectrochemical steady state convective-diffusion equation of the EC reaction in the flow cell was solved by the implicit difference method. The normalized absorbance of the intermediate R and the final product P were calculated. Several factors affecting the spectroelectrochemical behavior were analyzed, which were represented by the normalized parameter w and normalized rate constant K-1 respectively. The relationship between the normalized absorbence of intermediate R and w was discussed and a formula for calculating the rate constant of the following reaction was derived. The suitable range of application of the formula was discussed as well.