P-type (K, Fe) co-doped g-C3N4 was synthesized under self-generated NH3 atmosphere and displayed improved photocatalysis performances. Its physicochemical properties were comprehensively characterized. The reducing atmosphere, self-generated NH3, strongly affects photocatalytic performances, causing a downshift of the band alignments. Doping the g-C3N4 with K and Fe creates a higher degree of crystallization, more abundant porosity structure with higher specific surface area, lower band alignments and narrower band gaps with enhanced light absorption ability. Moreover, the photocurrent response and Mott-Schottky plots show that the as-synthesized (K, Fe) co-doped g-C3N4 is of p-type and has a boosted current density. Ultimately, the higher specific surface area, higher light absorbance capacity, lower recombination rate, and more photo-generated carriers lead to observed better photocatalytic performance. Photocatalytic activity evaluation in Rhodamine B solution could be well fitted by zero-order kinetic. Cycling experiments were employed to study the stability. Photocatalytic mechanism was investigated based on trapping experiments, suggesting a synergistic effect between low E-VB and p-type behaviors.