Abstract： The majority of the confirmed black hole binary systems in the Milky Way are soft X-ray transients, which are short-period binaries first identified during X-ray outbursts. It is usually challenging to measure the mass of the black hole for these systems as the optical emission from the companion is often contaminated by the contribution of accretion disk. In this talk, I will discuss how to reliably measure the black hole mass in soft X-ray transients using simultaneous spectroscopy and photometry, taking Nova Muscae 1991 as a case study.