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Based on the monthly 600 hPa ECMWF reanalysis data， the trajectories and characteristics of the lowlevel low pressure systems in active and weak monsoon years are analyzed， respectively. The results show that the low pressure system formed in April in the southwest of Qinghai Province and moved into Xizang along the southwest direction in May and then shifted westward with longitudinal fluctuations. After it reached the “farthermost west point，” it shifted back with attenuation and dissipated in October. The center intensities of the low pressure system in monsoon active years were stronger than those in monsoon weak years. The trajectories of the low pressure system were farther north than normal in active monsoon years and farther south with a Vshaped distribution in weak monsoon years with longitudinal fluctuations. The low pressure systems often formed repeatedly in monsoon weak years at the early stage. The low pressure systems over India formed later and more southslanting in active monsoon years than in weak monsoon years at the maturity stage. The high pressure system in the northwest of the low pressure system weakened at the later stage. In October， the eastern extension of the subtropical high ridge was farther east in weak monsoon years than in active monsoon years.