LnCO(3)OH/g-C3N4 (named Ln-CN, Ln=La, Pr) heterojunction was synthesized with a facile in-situ hydrothermal method by self-sacrificing a part of g-C3N4 as anion source. The photocatalytic activity of Ln-CN heterojunction was evaluated by nitrogen photofixation under simulated solar light irradiation. The result displayed that Ln-CN exhibited considerable improvement in nitrogen photofixation compared to g-C3N4 and LnCO(3)OH. Strong nitrogen adsorption energy offered by chemical adsorption ensure that more nitrogen was adsorbed and activated on the surface of Ln-CN. Moreover, in Ln-CN Z-scheme heterojunction, the electron at CB of LnCO(3)OH transferred to VB of g-C3N4 through the Ln-N interface channel. The recombination rate of charges carriers was reduced, leaving more electron at CB of g-C3N4 to reduce the adsorbed nitrogen. Results from time-resolved PL, photocurrent, and EIS characteristics further proved Ln-CN showed low recombination rate of photogenerated charges carriers. Finally, the stability of Ln-CN was certified to be excellent, which is significant for practical application. This study presents a simple self-sacrificial method to synthesize LnCO(3)OH/g-C3N4 heterojunction, which can be extended to the fabrication of other g-C3N4-based heterojunction.