As an important tool in microwave detection, quantum sensing techniques have been studied in a broad range of fields such as microwave electrometry and microwave magnetic field detection. In this work, we measure the relaxation rate of Rb atoms in vapor cells using a Rabi-resonance-based quantum sensing technique. The characterizations of the Rabi resonance line shape with different experimental parameters are investigated. The relaxation rate of the vapor cell can be obtained with the Rabi resonance line shape.