A microwave-modulated reflectance spectroscopy (MMRS) measurement system was constructed. This MMRS technique was used to identify a transition from holes in valence band to electrons in the ground subband (GS) of the two-dimensional electron system (2DES) formed in a GaAs/AlGaAs heterostructure sample. The temperature (T) dependence of the MMRS shows a blue shift of the energy gap with increasing T, while the magnetic field (B) dependence of the MMRS shows a red shift of the energy gap with increasing B. Both phenomena are attributed to the band-filling effect of holes in valence band in the GaAs/AlGaAs heterostructure. A theoretical simulation based on Kramers-Kronig relation was also presented which was similar with the experimental data.