In this paper, PDDA-functionalized black phosphorus nanosheets (BP) was used to construct BP/Ag2CO3 heterojunction photocatalyst by a simple co-precipitation process. The morphology, photoelectrochemical, and photocatalytic performance of BP/Ag2CO3 heterojunction photocatalyst were systematically characterized and analyzed. BP was proved to enhance the transport efficiency and restrain the recombination efficiency of the carriers in the BP/Ag2CO3 heterojunction. At the same time, BP greatly suppressed the Ag2CO3 photo-corrosion and improved the stability for methyl orange (MO) degradation. After four cycles, compared with the 19.2% degradation rate of MO in pure Ag2CO3, the photocatalytic activity remained as high as 87.1% in 15-BP/Ag2CO3 heterojunction. The Mott-Schottky and active species capture experiments enabled us to obtain the BP/Ag2CO3 heterojunction band gap configuration, and propose the reaction mechanism of the direct Z-scheme. The in-situ X-ray photoelectron spectroscopy (XPS) analysis and work function calculation verified the charge transfer in BP/Ag2CO3 direct Z-scheme heterojunction. This Z-scheme mechanism interpreted the enhanced photocatalytic stability of BP/Ag2CO3 for inhibiting the Ag+ photoreduction in the conduction band of Ag2CO3, and meanwhile elucidated the extremely improved photocatalytic activity by restraining the recombination of electron-hole pairs. This work is expected to be useful in providing a facile route for designing novel heterojunction photocatalytic material and carrying out high-efficiency wastewater treatment.