Nitrogen uptake by phytoplankton and nitrification mediated by nitrifying microorganisms in the upper ocean are key processes affecting marine productivity and carbon sequestration. Understanding how these two critical nitrogen cycle processes respond to the dual stressors of ocean acidification and warming represents a pressing research frontier in marine biogeochemical cycles and global change. Elucidating this issue will provide a theoretical foundation for accurately assessing future changes in ocean productivity and the efficiency of the biological pump. However, most existing studies rely on laboratory-based pure culture experiments, which may fail to adequately reflect the complex interactions between phytoplankton and nitrifying microorganisms in natural marine ecosystems and their responses to changes in environmental factors. This study systematically summarizes the impacts and mechanisms of ocean acidification and warming on nitrogen uptake and nitrification. In addition, more attention needs to be paid to other factors, such as strengthened ocean stratification and decreased dissolved oxygen contents, induced by ocean acidification and warming, which could indirectly affect nitrogen uptake and nitrification. Existing problems such as insufficient in-situ monitoring of ecosystems, limited synergistic studies on multiple processes and stresses, and inadequate understanding of long-term adaptation processes, are highlighted. Finally, three key areas are proposed for future research: \mathrm{①} synchronous coupling analysis of nitrogen uptake and nitrification processes, and clarifying the interactive effects of acidification and warming, \mathrm{②} exploring the vertical differentiation response mechanisms of the above processes in the upper ocean, particularly in oligotrophic oceans, where critical knowledge gaps exist, and \mathrm{③} elucidating the long-term adaptation processes and nonlinear responses of phytoplankton and nitrifying microorganisms. A three-in-one research framework is constructed—encompassing the spatial dimension, temporal scale, and the experimental system—to provide a scientific basis for evaluating the evolution of key nitrogen processes and marine productivity under global change.