Precipitable water vapor (PWV) with high precision and high temporal resolution can be obtained based on the global navigation and positioning system (GNSS) technique, which is important for GNSS in disaster prevention and mitigation. However, the related studies on drought monitoring using PWV are rarely performed before, which becomes the focus of this paper. This paper proposes a novel drought monitoring method using GNSS-derived PWV and precipitation, and a multi-time-scale standardized precipitation conversion index (SPCI) is established. This index is different from the traditional index in terms of expression, standardization and time scale. The proposed SPCI is then compared with SPI/SPEI/scPDSI (Standardized Precipitation Index/Standardized Precipitation Evapotranspiration Index/ self-calibrating Palmer Drought Severity Index) and applied to local and global drought monitoring. Validated results show that multi-time-scale SPCI has good consistency with the corresponding SPI/SPEI/scPDSI. The correlation between SPCI and SPEI is the strongest (more than 0.96) in 12-month scale, which indicates the application potential of SPCI in drought monitoring. In addition, applications for regional (Queensland, Australia) and global drought/wet monitoring further verify the capability of the proposed SPCI. The average percentage deviations of drought/wet monitoring between SPCI and SPEI are 2.77% and 3.75%, respectively on a global scale. The above results show that the SPCI developed in this study is efficiently applied to global flood/wet studies.