From the time series of Climate Forecast System Reanalysis (CFSR), rain gauge data, and case studies, two widespread heavy rainfall (> 80 mm day^-1) periods over Taiwan during the South China Sea Two Island Monsoon Experiment (SCSTIMX) (1 to 4 June and 14 to 18 June 2017) are found to be closely related to the large moisture transport within the marine boundary layer (MBL) from the northern South China Sea [integrated vapor transport (IVT) between surface and 900-hPa level > 220 kg m^-1 s^-1] to the Taiwan area. With most of the moisture confined within the boundary layer, the moisture transport to the Taiwan area mainly occurs in the marine boundary layer jet (MBLJ). For both periods, the synoptic system-related low-level jet (SLLJ) coexists with the MBLJ, which is a subsynoptic feature. The MBLJ develops and intensifies when the mei-yu trough over southern China deepens and/or the western Pacific subtropical high strengthens and extends westward. With significant upstream moisture transport within the MBL (IVT ~300 - 315 kg m^-1 s^-1), extreme torrential rain (> 500 mm day^-1) occurs over Taiwan during 2 to 3 June of the first widespread heavy rainfall period. During the second widespread heavy rainfall period, there are two sub-periods of MBLJs and rainfall peaks (> 300 mm day^-1) on 14 and 17 June with lower moisture transport by MBLJs (IVT ~220 - 280 kg m^-1 s^-1) than during the first heavy rainfall period. For both periods, the moisture-laden MBLJs lifted by terrain and/or mei-yu jet/front systems produce heavy rainfall. The moisture transport within the MBL from the northern South China Sea to Taiwan provides a useful guide to predict heavy rainfall over Taiwan.