The anomalous moving track of Typhoon Prapiroon is studied by using th e NCEP Global Data Assimilation System （GDAS） data and CIMSS microwave brightness tem perature data. The results show that the northward-deflecting track of Prapiroon is closely related to the midlatitude westerly trough, southwest monsoon and typhoon asymmetric structure itself. The meridional wind activities behind the w esterly trough and the directiron of maximum wind speed center have a certain in dication meaning for the moving direction of typhoon.