Supramolecular self-assembly and chemical precipitation methods were used to prepare P-doped three-dimensional graphitic phase carbon nitride (g-C3N4) MNP-x using melamine and cyanuric acid as raw materials and ammonium phosphate as a P-dopant (x denotes the added mass of ammonium phosphate when the melamine is 0.01 mol, the same below), and characterisation of MNP-x was performed by XRD, FTIR, XPS, SEM, UV-Vis and photoluminescence (PL), etc. The photocatalytic degradation of the dye rhodamine B (RhB) reaction was applied to evaluate the photocatalytic performance of MNP-x. A free radical trapping agent was used to verify the reactive groups that play a role in the photocatalytic reaction and to speculate the reaction mechanism. The results showed that MNP-0.08 prepared under optimal conditions, compared to the undoped P-doped g-C3N4(MN), SEM showed that the MNP-0.08 morphology changed from a one-dimensional tubular to a three-dimensional bouquet; XPS confirmed that the P atoms replaced some of the C atoms doped into the g-C3N4 structural unit; PL showed that MNP-0.08 improved the electron-hole pair (e-,h+) separation; UV-Vis showed that MNP-0.08 changed the energy band structure, the valence band (VB) from 1.96 eV to 2.00 eV, and the conduction band (CB) from 0.7 eV to 0.75 eV,,and the energy band gap value (Eg) from 2.66 eV to 2.75 eV; it was verified that -O2- as the main reactive group makes the MNP-0.08 photocatalytic degradation of RhB was improved with a reaction rate constant (k) of 0.02816 min-1, which was 2.95 times higher than that of MN (k=0.00955 min-1); the synergistic effects of the change in energy band structure, P as an electron-rich centre to promote carrier separation, and P doping to induce a high charge transfer and localize excited electrons made MNP possess a much higher photocatalytic performance than MN with higher photocatalytic performance.