Photocatalytic technology is considered to be one of the most prominent strategies to address energy and environmental problems by utilizing visible light. As a metal-free semiconductor, graphitic carbon nitride (GCN) has attracted global research attention due to its low toxicity, stability, versatile 2D structure, and phenomenal visible light activity. Higher recombination ratio and poor separation efficiency limited the practical applications of GCN. Recently, the concept of g–C3N4–based homojunction became a research interest due to its phenomenal separation efficiency and suppressed recombination of electron-hole pairs. Based on the charge transfer mechanisms, the g–C3N4–based homojunction predominantly followed type-II, Z-scheme and S-scheme mechanisms. This review describes the construction of GCN-based homojunction towards energy and environmental remediation, including organic pollutant degradation, hydrogen production, carbon dioxide reduction, and hydrogen peroxide production. A detailed emphasis is given to the different types of GCN-homojunction and the charge transfer pathways. Finally, the advantages, disadvantages, and future perspectives of GCN-based homojunction photocatalysts are explained.