Base on the conventional observational data and NCEP 1°×1° reanalysis data, the causes of low trough and coldfront rainfall processes without vortex or low level jet in the lower troposphere, one influenced by a backwardtilting trough and the other by a forwardtilting trough, are analyzed contrastively. The results show that although the circulation patterns of two processes in the lower troposphere were very similar, there were significant differences on the spatial structure. The front of the backwardtilting trough tilted toward the cold air and was divided into three sections. The cold air of the first section below 850 hPa was weak, but played a triggering role for the precipitation process. The rainfall distribution was on the leading edge of the first section in its moving direction, where the dense streamline zone in cyclonic convergence was on the ground. The front of the forwardtilting trough was complete, a wet baroclinic frontal zone inclined to the warm area. The rainfall distribution was 1 to 2 latitude distance in front of the front where the dense streamline zone in southward airflow was on the right side of the ground convergence line. Both heavy rain events had strong water vapor transport at lower levels, and there existed high temperature and high humidity, with humidity up to 15 g/kg at 925 hPa. The difference is that the rainfall area of the former was in the highvalue water vapor flux area in the front edge of the dense 〖WTBX〗θ〖WTBZ〗e lines in wind convergence region; while the rainfall area of the latter was in the dense water vapor flux isopleth zone at the tip of 〖WTBX〗θ〖WTBZ〗e warm tongue in wind convergence region, but more inclined to the warm air side. In addition, heavy rains often occurred in such special orographic uplift areas as mountainous areas or coastline.