In this study, we have constructed ZnCr-LDH/g-C3N4 composite photocatalysts via in-situ synthesis approach. This composite material was used as catalyst interlayer of BPM to promote water dissociation. Results indicated that the combination of ZnCr-LDH and g-C3N4 could effectively improve the photoelectrocatalytic water dissociation activity. When the experiment time lasted for 1 h, the pH decreased to 1.52 in compartment 2, and increased to 13.11 in compartment 4 by using ZnCr-LDH/g-C3N4 composite catalyst.