Light and pH are key factors regulating biological iron redox process in soil anaerobically. Samples of paddy soil were prepared into slurry using deionized water and pH buffer, and then incubated anaerobically under dark and, illumination and constant temperature, to investigate variation of soil pH in the process and effect of pH on iron redox cycle. The relationships of pH with, Fe(II) and water soluble inorganic carbon during the anaerobic incubation were also explored. Results showed that soil pH could be altered by illumination during the anaerobic incubation, decreasing under dark, and rising and then declining under illumination, with time elapsing. Iron reduction occurred when pH was in the range from 4 to 9, and peaked at pH 7, reaching 128.5 μmol g^-1, and it was inhabited under dark when pH was adjusted to 4 or 9. Under illumination, Fe(II) produced from iron reduction was re-oxidized when pH was in the range of 6~8. The re-oxidation was enhanced up to 77.13%, or 49.17 μmol g^-1 when the buffer initial pH was 7. A significant linear positive relationship was observed between Fe(II) and WSIC during the anaerobic incubation under dark, but significant negative ones were of pH with Fe(II) and water soluble inorganic carbon (WSIC).