The invention discloses a carbon potential calculating method. The carbon potential calculating method comprises the steps: 1) determining an input analog signal for carbon potential calculation; 2) sampling the original input analog signal; 3) performing arithmetic average filtering for the original input analog signal; and 4) calculating the carbon potential. As the carbon potential calculating method calculates the carbon potential after performing segment arithmetic average filtering processing for the original input analog signal, the technical problem that the carbon potential is great in fluctuation and the data is unstable because the analog input signal is interfered by the external environment is effectively solved. And at the same time, the carbon potential calculating method can reduce the number for carbon potential calculation and save the calculation resource for PLC.