Nozzle internal flow dictates the spray breakup, fragmentation and combustible mixture preparation. The dynamics of nozzle internal flow and the closely followed primary breakup under various injection pressures were studied by employing high-speed microscopic imaging technique. The effects of interaction between split injection by varying dwell interval and distribution of energizing duration between split injections on the nozzle internal flow were also investigated by using multiple injection strategy. It was found that the initial in-nozzle condition and injection pressure governs the spray tip morphology and breakup process through the strength of interaction between liquid fuel and air bubbles. Throttling effect introduced a large cloud of cavitation vapor bubbles at the needle seat during the initial stage and when stage when effective injection pressure is low. During steady flow stage, high pressure suppressed the initiation of cavitation through throttling but enhanced the cavitation generation through flow redirection. When multiple injection strategy is employed, cavitation was overall weakened, especially for flow redirection at the nozzle inlet due to reduced effective injection pressure. The injected fuel mass was reduced, and breakup quality deteriorated significantly, especially under low injection pressure. It is therefore unwise to use closely coupled split injection strategy under low injection pressure. This studied is believed to be useful for injection control and related modeling.