Constitutive mixotrophs (CM), combining phagotrophy with inherent phototrophy, are critical primary producers and consumers in marine pelagic food web. However, how mixotrophs, especially obligate CM, balance phototrophy and phagotrophy in response to changing environments remains equivocal. Here, we examined the photosynthesis traits and phagotrophic activities of a mixotrophic green alga, Picochlorum sp. GLMF1 over a wide range of temperature-irradiance combinations. We found that photosynthetic electron transport rates (ETR) and chlorophyll-a specific inorganic carbon uptake rates increased with increasing irradiance, albeit at different paces at different temperatures. The maximum ETR (p_max) derived from the P-I curve decreased from 22.36 ± 0.74 μmol e m^-2 s^-1 at 22â„?to 9.10 ± 0.13 μmol e m^-2 s^-1 at 31â„? suggesting a significant constraint of photosynthesis at high temperature. By contrast, the ingestion of Picochlorum sp. increased with increasing temperature in most cases, which helped the obligate CM grow better in some photosynthesis-limited conditions, such as low irradiance and high temperature. Our results showed the flexible nutritional strategy of Picochlorum sp. in response to temperature and light. Increasing light favors photosynthesis, whereas warming benefits phagotrophy. Such complementation renders them flourishing in a wide range of environmental conditions, which potentially endows them a competitive edge in changing ocean. Our study adds knowledge to the nutritional strategy of mixotrophic green algae, with significant implications on their ecological roles in the marine food web under the projected climate changes.