In this paper, the characteristics of the short-time scale variation of the East Asian Subtropical Upper-level Jet Stream (EASJ) were studied by use of the high spatial-temporal resolution simulation data output of the nonhydrostatic mesoscale model WRF. The possible mechanism for the EASJ intensification is explained in light of the atmospheric motion equation. The results indicate that the horizontal scale of zonal wind increment is greater than 2 000 km in the 6 h time interval. Few high-frequency disturbances occur to the north side of EASJ axis, while many occur to the south. Correspondingly, the intensity of zonal wind on the north side of the EASJ axis increases in a large scale, while a negative value area with a horizontal scale of several hundred kilometers still exits in the positive value area of zonal wind increment on the south side of the EASJ axis. The period of high frequency disturbance is about 1-6 h, and the eastward propagation speed is 20-40 m·s^-1, which is similar with that of the inertial gravity wave. The main physical factors that contribute to the short-term scale increase of EASJ are horizontal advection and ageostrophic effects. The magnitude of the horizontal advection term is the same as that of the ageostrophic term. However, the variation trends of those two factors are basically opposite. The magnitude of the sum of the two is similar to the local acceleration term of the zonal wind. In addition, the vertical advection also contributes to the change in EASJ intensity.