A greenhouse pot experiment simulated wetland conditions to study vetiver grass(Chrysopogon zizanioides(L.) Roberty) growth and photosynthetic responses under cadmium(Cd, 0.3 mg/kg) and sulfamethoxazole(SMX, 10 mg/kg) composite pollution. Five phosphorus levels(60, 120, 180, 240, and 360 mg/kg) were tested, alongside a control(CK) without pollutants and phosphorus. The experiment lasted 42 days, with measurements taken on days 1, 2, 3, 5, 7, 14, 28, and 42, including the fresh biomass of the aboveground and belowground parts of vetiver grass, chlorophyll content, photosynthetic parameters, and chlorophyll fluorescence parameters. The objective was to investigate the effects of exogenous phosphorus on vetiver grass growth and photosynthetic characteristics under Cd-SMX composite pollution. Early stages(days 1-7) showed a “toxicity stimulation effect” with increased net photosynthetic rate(Pn) and apparent light use efficiency(LUE) under composite pollution. However, Cd and SMX accumulation later caused toxic effects and reduced photosynthetic capacity, with key parameters including the actual photochemical efficiency of PSII, maximum photochemical efficiency of PSII(Fv/Fm), potential photochemical efficiency(Fv/Fo), electron transport rate, and photochemical quenching coefficient, significantly lower than those in the CK on day 42. Phosphorus’s “growth-promoting effect” helped maintain the photosynthetic stability of vetiver grass, with Pn, Fv/Fm, Fv/Fo, apparent CO2 utilization efficiency, and LUE recovering to levels comparable to day 1 by day 42. Notably, Pn in the CS4 treatment reached 19.55 μmol/(m2?s), significantly higher than in other treatments. Meanwhile, the MDA content in the stems and leaves of vetiver grass under the CS4 treatment was the lowest under composite pollution, measuring 0.93 nmol/g. In summary, Cd-SMX composite pollution weakened vetiver grass’s photosynthetic capacity, but 240 mg/kg phosphorus enhanced its growth of aboveground stems and leaves and photosynthetic resilience, improving tolerance to Cd-SMX stress.