The High Energy cosmic-Radiation Detection facility (HERD) is a calorimetry-based cosmic-ray detection mission on board China space station. A side-on transition radiation detector (TRD) focuses on the TeV energy range calibration of the HERD calorimeter with an error of less than 10% by measuring the Lorentz factor γ of high-energy cosmic-ray protons. A side-on TRD prototype was developed for the performance study and tested by Deutsches Elektronen-Synchrotron electron beams. In this paper, the Monte Carlo simulation based on GEANT4 was compared with the experiment results. All simulations were in good agreement with the test beam results.