Layered transition metal dichalcogenides (TMDs), as a new class of two-dimensional material, have received wide attention of scientific community due to their peculiar electronic and optical properties. Monolayer TMDs such as MoS2, MoSe2, WS2 and WSe2 are semiconductors with band gap energies in the visible and near-infrared region, which promises the applications in logic nano-devices, ultra-high speed photoelectric detectors and nano-lasers. Temperature has strong influences on the electronic and optical properties of semiconductors, and their applications in photonic and optoelectronic devices. Thus, the research on the temperature dependence of the energy band of monolayer TMDs is important and meaningful. Monolayer MoS2, as a prototype of TMDs, displays a weak absorption line with a strong background in original reflection or absorption spectra. The strong background has a tremendous influence on the determination of excitonic transition energy and linewidth. In this work, we adopt the reflection magnetic circular dichroism (MCD) spectroscopy in which reflection spectra and MCD spectra can be simultaneously obtained. We demonstrate that the background disturbance is eliminated in the MCD spectra, in contrast to the reflectivity spectra. And we discuss the optimization of our home-built experimental setup in detail. Through the elaborate analysis of the MCD theory, we demonstrate that the excitonic transition energy and linewidth can be directly and accurately extracted from the MCD spectrum. We perform the reflection MCD measurements on monolayer MoS2 in a temperature range of 65–300 K. The transition energies and linewidths of A and B excitons of monolayer MoS2 are extracted, respectively. Those functional parameters that describe the temperature dependence of the energy and linewidth of both excitonic transitions are evaluated and analyzed. We find that the broadening of the linewidth is related to the LO phonon scattering, and the linewidth of A exciton is clearly narrower than that of B exciton. The linewidth difference between A and B excitons might result from the stronger inter-valley coupling of B exciton. Our results indicate that MCD spectroscopy, as a modulated spectroscopy by magnetic fields, provides an easy tool to determine the features of monolayer excitons.