Abstract: The cloud characteristics of a convective rain band that occurred on the northwest edge of the western Pacific subtropical high（WPSH） on August 25, 2009 are analyzed using infrared cloud images. Using the ARW-WRF V3.3 model, the occurrence and development of the convective rain band are simulated. Based on the success of the simulation, the symmetric, convective, inertial instabilities, and frontogenesis of the convective rain band are studied using the model output. The results show that the convective rain band on the northwest edge of the WPSH consists of a number of mesoscale convective cells, which develop and die out when moving with the middle troposphere environmental flow and often bring heavy rain in their passing-by region. There is a broad baroclinic cloud band on the northwest side of the band and a cloudless sky controlled by WPSH on the southeast side. The convective band occurs in the symmetric unstable area below 700 hPa and there are convective and weak inertial instabilities between 700 and 500 hPa. With the development of the convection, the convective instability becomes weak, but the inertial instability becomes intensified between 700 and 500 hPa. The convective body is an obvious mixture of slanted convection and vertical convection, with slanted upward flow in the low troposphere, vertical upward flow in the mid–high troposphere, and downward flow on the left side of the body, reflecting the energy release of convective-symmetric instabilities. The frontogenesis below 750 hPa provides an advantage for energy release of convective-symmetric instabilities. The band is nearly parallel to 800–500 hPa constant thickness lines, which deviates from 500 hPa contour lines. The convective body in the rain band moves with environmental flow, showing that the rain band is in accordance with banded precipitation characteristics, which is associated with symmetric instability. The above conclusions can be used for operational forecast on the location and orientation of the convective rain band on the northwest edge of WPSH.