Episodic accretion has been observed toward many young protostars, mostly Class II young stellar objects (YSOs). Since a large fraction of its final mass accretes onto the central protostar during the early stage of star formation, it is essential to study the episodic accretion process within embedded YSOs. The Class I YSO, EC 53 in the Serpens Main cloud, has repeatedly undergone robust flux enhancements at 850 μm. Such sub-mm variations are expected to be related to accretion burst events. In this study, we perform two- and three-dimensional continuum radiative transfer modeling to quantify the fractional internal luminosity rise in EC 53 corresponding to the observed flux enhancements. We model the spectral energy distribution and radial intensity profile both in the quiescent and outburst phases. The effect of the external heating from the interstellar radiation field is also considered. For EC 53, we find that the role of external heating from interstellar radiation field appears insignificant. The radial intensity profile analysis clearly demonstrates that the detected sub-mm flux variation of EC 53 comes from the heated envelope by the accretion burst.