Abstract: There is a close relation between the variation of middle troposphere synoptic system over the Qinghai-Xizang Plateau and movement of typhoon over West Pacific, and the relation is studied with the observation study and dynamical diagnosis from the point of view of downstream development and interaction between mid-latitude and low latitude. 25-year statistics results from 1970 to 1995 indicate that the relations between middle troposphere circulation patten over the Plateau and the track of typhoon over West Pacific are as follows: When low-pressure system on 500 hPa isobaric surface occupies middle troposphere over the Plateau, it is beneficial to typhoon’s Westward movement; on the contrary, high-pressure system on 500 hPa isobaric surface is always beneficial to typhoon recurvature. The results of dynamical diagnosis show the physical mechanism of impact of synoptic system over the Plateau on downstream synoptic system, that is, the transport of disturbance kinetic energy over the upstream Plateau makes the downstream trough developed. And then it affects the steering flow of typhoon. Because of existence of the ridge over the Plateau, the eddy kinetic energy transport channel moves to the north. On the contrary, trough exists over the Plateau, so the eddy kinetic energy transport channel moves to the south. The southerly wind in front of the trough over the Plateau together with the southerly wind at the east part of typhoon transports low potential vorticity of low latitude into subtropical high. That is beneficial to development of subtropical high, and then subtropical high affects typhoon’ s movement. These results fully reflect the impact of interaction between mid-latitude and low latitude on synoptic system