An analysis is made for a special storm occurred on Aug.25, 1982 in the southern part of the Yellow Sea. It is an oceanic semi-tropical cyclone, directly occured on the surface of higher latitude ocean and accompanied with strong wind and heavy rain. The upper air divergence of troposphere,positive vorticity advection and lowlevel southeast jet make the mechanism responsible for the system's occurrence and development and the latent heat of condensation released from the convection contributes greatly to a further development and strengthening of it.