In order to study the echo mechanism of Mesosphere-Stratosphere-Troposphere(MST) radar at tropospheric height, the phenomenon of abnormal echo power in the upper troposphere is studied by using the radar echo power and wind field under special weather conditions(cut off low pressure), Lomb-Scargle spectrum analysis, harmonic fitting, Richardson number and other methods. Experimental and simulation results show that under the cutting off of low-pressure weather conditions, persistent gravitational waves are generated by the jet stream and deep convection, and the gravity waves are disseminated from the bottom up beneath tropopause and saturated broken, which dissipates energy in the atmosphere, destroys atmospheric static stability, and forms the Kelvin-Helmholtz(K-H) instability layer, and then develops into turbulence, thus making the region of radar echo power increased.