With commercial red phosphorus (C-RP) as the raw material and NaOH as the regulator, the HRP-pHx (x=5, 7, 9) photocatalyst was obtained by hydrothermal treatment. The photocatalytic activity of HRP-pHx was evaluated by photodegrading Rhodamine B (RhB), in which HRP-pH7 manifested the highest photocatalytic activity. Its rate constant for RhB was 3.1×10^-2, which was 2.8 times that of HRP. The series characterization results showed that the main reason for the RP photocatalytic activity enhanced by adjusting pH is that the uniform structure and the smaller particle size make HRP-pH7 have more active sites than HRP. The repeated experiments indicated that HRP-pH7 was a stable photocatalyst for degrading RhB (the degradation rate was over 92% for reusing the catalyst the fourth time). In active species trapping experiments it was found that the photogenerated holes and superoxide free radicals played a main role in degrading RhB.