The tropical cyclones (TCs) that occurred during 2002-2011 in the northwestern Pacific Ocean and the connection between the moisture and their ability to subsequently undergo rapid intensification (RI) are investigated with the NCEP FNL (Final Analysis) data and the best-track datasets of the JTWC (Joint Typhoon Warning Center), which is organized into 24-h intervals of either RI and slow intensification/constant intensity (non-RI) periods. The environmental and internal distributions of moisture and its transport at each interval are statistically analyzed to reveal their significant influence on the intensity change in the subsequent 24 hours. The results show that relative humidity area-averaged from the radius of 3° to 10° latitude at 900 hPa (RH_3-10) can clearly distinguish RI from non-RI, which means the sensitive vertical level of moisture in the northwestern Pacific Ocean is lower than that in the Atlantic Ocean. In addition, larger RH_3-10 values and smaller horizontal moisture flux (F_all) mark the beginning of RI. With the consumption of the convection development in RI, the environmental humidity decreases and the moisture flux is enhanced. The obvious differences of the moisture distribution and transfer between the TC inner-core and outer-rainband region are also studied, showing that the net convergence region of moisture flux in RI is larger than that in non-RI. The correlative analysis also indicates that intensity forecasts and RI predictions in particular may be aided by the use of the moisture and its flux, both in the environmental and internal region.