The Belle II experiment at the High Energy Accelerator Research Organization (KEK) in Japan started gathering data in Phase II in April 2018 to unravel new physics beyond the standard model by precisely measuring charge conjugation parity symmetry (CP) violation and rare weak decays of heavy quarks and leptons. It was performed at the SuperKEKB electron-positron collider mainly running at the <inline-formula> <tex-math notation="LaTeX">\Upsilon (4S) </tex-math></inline-formula> resonance energy with the goal to reach the maximum instantaneous luminosity of <inline-formula> <tex-math notation="LaTeX">8\times 10^{35} </tex-math></inline-formula> cm −2 s −1 . A new algorithm is needed to operate the Belle II calorimeter trigger system stably in the much higher luminosity and beam background environment of SuperKEKB compared with the KEKB collider. In order to develop an appropriate algorithm, a detailed simulation study of the Belle II calorimeter trigger system is crucial. In this article, we report the results of the simulation of the electromagnetic calorimeter (ECL) trigger using physics and beam background Monte Carlo (MC) events and compare them with the ECL trigger performance in the Phase II operation. The simulation package is developed with the Belle II Geant4-based analysis framework called Basf2.