Photocatalysis is an efficient and eco‐friendly method of solving the environmental problems, but the low charge separation efficiency of photocatalysts limits their application. Herein, we report the fabrication of CeO 2 /g‐C 3 N 4 composites via an in‐situ growth of CeO 2 on the dispersed g‐C 3 N 4 sheets (CN‐OH) in an strong KOH condition, and the resultant CeO 2 /CN‐OH composites set up a new interface structure, which demonstrate a dramatically boosted the photocatalytic activity compared with CeO 2 /g‐C 3 N 4 composites by conventional two‐step combination method. The decomposition efficiency of CeO 2 /CN‐OH for 2,4‐dichlorophenol (2,4‐DCP) degradation comes up to 6 times of bare CeO 2 or g‐C 3 N 4 . Compared to conventional CeO 2 /g‐C 3 N 4 , the CeO 2 /CN‐OH showed 65.9% and 23.2% improvement for (2,4‐DCP and Cr(VI) removal, respectively. The significantly enhanced photodegradation ascribe to the alkaline modified CN‐OH and the fabrication of CeO 2 /CN‐OH n‐n type heterojunction, which enables to improve the charge separation efficiency and increase the reaction active sites. Furthermore, the probable pathway and concreted photocatalytic mechanism for 2,4‐DCP degradation are proposed based on the detected intermediate products.