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This study aims to explain the rapid intensification (RI) onset of Typhoon Megi from a new perspective of the intraseasonal and interannual sea surface temperature anomaly (SSTA) effect. Simulation result indicates that the warmer intraseasonal SSTA along tropical cyclone (TC) track seems to be more conducive to the early occurrence of TC RI onset than the interannual SSTA for Megi. For the experiment with intraseasonal SSTA (SST-intraseasonal), it has cooler averaged SST in TC inner-core, outer-core, and external environment than the experiment with interannual SSTA (SST-interannual). Cooler TC outer-core and environmental SST provide more midlevel radial inflow, resulting in a strong TC lower-level warm core in SST-intraseasonal. For SST-interannual, the stronger upper-level VWS related to the warmer environmental SST strongly tilts TC structure, delaying the time of RI onset. Conceptual model of Megi RI onset proposed that, when TC lower-level warm core gradually weakens and TC upper-level alignment is basically completed, TC RI onset occurs. Our results underscore the effect of intraseasonal SSTA on Megi RI onset, and provide a new perspective for improving the prediction of TC RI onset. [ABSTRACT FROM AUTHOR]