While the marine N₂-fixers Trichodesmium spp. are under influences of increasing pCO₂ and ultraviolet radiation (UVR) in their habitats, little is known on their potential responses to future ocean acidification in the presence of UVR. We grew Trichodesmium at two pCO₂ levels (410, 1000 μatm) under sunlit conditions with UVR, documented the filament length, growth and chlorophyll a content after its acclimation to the pCO₂ treatments, and then measured its carbon and N₂ fixations under different solar radiation treatments with or without UVR. We showed that the elevated pCO₂ did not significantly alter the diazotroph’s growth, filament length and pigment content, and its photosynthesis was only affected by solar radiation treatments rather than the pCO₂ levels. Presence of UV-A and UV-B inhibited the photosynthesis by 10%-22% and 17%-21%, respectively. Inhibition of N₂ fixation by UV-B was proportional to its intensity, whereas UV-A appeared to stimulate the N₂ fixation at low but inhibit it at high intensities. The elevated pCO₂ only stimulated N₂ fixation under moderate levels of solar radiation. The simulation of the depth profile of N₂ fixation in the water column showed that UV-induced inhibitions dominated the combined effects of elevated pCO₂ and UVR at shallow layer (0-30 m) and their combination enhanced N₂ fixation at 30-60 m depth, which faded at deeper layer. Altogether, our results suggest that Trichodesmium could be influenced more by UVR than pCO₂ and their combination results in negative under high but positive effects under low to moderate solar radiations on its N₂ fixation.