Constructing heterojunction structure is of great significance for enhancing photocatalytic activity. Herein, a novel layered PI/g-C3N4 heterojunction was successfully prepared by a simple chemical adsorption method. The photocatalytic activity of PI/g-C3N4 heterojunction was determined by degradation of rhodamine B (RhB) under visible light. The 20PICN composite photocatalyst degraded RhB by 99% within 2 h, while pure g-C3N4 degraded RhB by only 69.9%. Moreover, the kinetic constant of 20PICN composite photocatalyst was 3.8 times that of g-C3N4. The 20PICN composite photocatalyst had excellent cycle stability. After five cycles of testing, the crystal structure had not changed and the ability to degrade RhB had not significantly decreased. It was found that h(+) and center dot O2- were the main active species in the degradation process. The possible mechanism of PI/g-C3N4 degradation of RhB was explored. The formation of heterojunction improved the separation efficiency of photogenerated carriers, thereby enhancing its photocatalytic activity. This study provides a reference for the industrial production and application of g-C3N4-based metal-free photocatalyst.