A facile step-by-step synthetic route was developed to synthesize the three-dimensional (3D) BiOI/g-C3N4 heterojunction photocatalyst by loading BiOI nanosheets onto g-C3N4 microtubes. And adjusting the material ratio easily controlled the amount of BiOI nanosheets on BiOI/g-C3N4 photocatalyst (BiOI/CN-0.5, BiOI/CN-1, BiOI/CN-3 and BiOI/CN-7). The photocatalytic properties were investigated by degradations of tetracycline (TC) and p-chlorophenol (4-CP) with visible-light irradiation. Eminently, the BiOI/CN-1 photocatalyst exhibited the best performance with the degradation rate of 86.98% TC in 90 min (the degradation rate 68.95% 4-CP in 240 min), which was much higher than pure BiOI or g-C3N4 as photocatalyst. Moreover, the BiOI/CN-1 photocatalyst possessed high stability and could be reused without much decrease in photocatalytic activity after four recycles. The significant separation and transfer property of photogenerated charge carriers were identified by photocurrent and PL spectroscopy, which contributed to the excellent degradation character of BiOI/CN-1 photocatalyst. In addition, active species capture experiment showed that the h(+), OH and O-2(-) were active radicals in the degradation process. And then, the Z-scheme transfer process of photogenerated charges for promoting catalytic performance was unveiled.