Abstract: The diag nosis of the short-term structure of the subtropical anticyclone over the Western Pacific(SAWP) in the summer of 1998 shows that, other than uniform descent, ascent at 500hPa often exists with in the area of SAWP.These features are determined by the complicated dynamic and thermodynamic structures of the SAWP.Under the 500hPa SAWP ridge, along the ridge line in the planetary boundary layer, there always exist divergence and descent, which greatly accounts for the usual clearness in the SAWP area.This complicat ed structure was also studied by employing atmospheric dynamic theory.It was found that vertical wind shear over the west part of the SAWP that is forced by external forcing should be accompanied by ascent.On the other hand, the negative vorticity advection at higher levels is the major forcing for the appearance of descent over the west of the SAWP.