The structural modulation of graphitic carbon nitride (g-C₃N₄) is regarded as an effective strategy to boost its photocatalytic behavior. Herein, we have synthesized 4,6-dimethyl-2-hydroxypyrimidine-doped and cyano defects co-modified graphitic carbon nitride (HDMP–CD-g-C₃N₄) using a facile in situ co-condensation method. HDMP–CD-g-C₃N₄ was employed for the degradation of tetracycline hydrochloride, exhibiting enhanced photocatalytic activity and catalyst stability. The characterization results demonstrate that the doping of HDMP decreases the band gap, thus promoting the light absorption of g-C₃N₄. Meanwhile, as an electron acceptor, HDMP broadens the π-conjugated off-domain system and accelerates the transfer of photogenerated electrons. In addition, the introduction of cyano defects further improves the separation efficiency of photogenerated electrons and holes. Analysis of time-resolved fluorescence spectra solidly supports these conclusions. Therefore, the photocatalytic degradation rate of HDMP–CD-g-C₃N₄ was 3.8 times that of the original g-C₃N₄, 2 times that of HDMP-doped g-C₃N₄ (HDMP-g-C₃N₄) and 2.5 times that of cyano defects-modified g-C₃N₄ (CD-g-C₃N₄). This work provides a way for collaboratively modulating the structure of g-C₃N₄ with molecular doping and defects, which is expected to bring novel prospects for the application of g-C₃N₄ in the treatment of waste water.