Phosphorus (P) is an essential nutrient element of phytoplankton, as well as a limiting factor for phytoplankton growth. It controls the succession of blooms from diatoms to dinoflagellates, especially in the East China Sea (ECS), where the eutrophication is serious. Most studies have only considered a single aspect of dissolved inorganic phosphorus (DIP) or dissolved organic phosphorus (DOP). In contrast, we investigated the growth interactions among the major bloom-forming marine diatom Skeletonema costatum and dinoflagellates Prorocentrum donghaiense and Karenia mikimotoi by using bi-algal cultures combined with DIP and DOP. Our results revealed that (1) P. donghaiense and K. mikimotoi have survival strategies that are superior to those of S. costatum in a bi-algal culture, whether under P-sufficient or P-deficient conditions, and (2) P. donghaiense has a slight competitive advantage over K. mikimotoi when P is sufficient, but the reverse is true when P is deficient. The difference in interspecific competition results at different P concentrations with DIP and DOP mainly arises from the variation in the utilization abilities of different species as regards different P sources, a finding which can also provide strong evidence for revealing the succession of diatoms and dinoflagellates blooms in the ECS.