Horizontal advection and diabatic heating terms in the potential vorticity tendency equation are used to diagnose the twice track changes of typhoon Muifa based on ECMWF data. The results show that horizontal advection is about one order of magnitude larger than diabatic heating; large-scale steering flows represented by horizontal advection mainly contributed to the first track change; horizontal advection and diabatic heating both contributed to the second track change, while horizontal advection controlled the direction and diabatic heating restrained its track recurvature.