In this paper, the characteristics of atmospheric boundary layer for the afternoon graupel process on 29 August 2016 are analyzed by making full use of densified observation data of sounding balloon and reanalysis data from ECMWF’s ERAInterim 0.125°×0.125° and comparing to the typical sunny cases on 26 August 2016. The results show that the temperature zero line increases with time and the stratification of temperature lapse rate is remarkable. The inversion layer is not obvious and the boundary layer has more unstable convection stratifications. Potential temperature increases with height and it presents an increasing trend of 5 K·(2 h)-1 with time. Specific humidity decreases as height increases and the content of water vapor is greater than that in sunny days. Wind speed varies with height. The wind speed in the near surface layer is higher than that at the same altitude in sunny days and the top wind velocity of boundary layer is less than that of sunny boundary layer. The wind direction is always dominated by west wind and it does not fluctuate greatly with height. Cloud coverage before graupel is large and cloud thickness reached 4000 m, there are complex vertical motions. The surface layer is descending while clouds are ascending. There is obvious supercooled water in the boundary layer at 08:00 BT before graupel on 29 August in Nagqu, and the top of the boundary layer fluctuates enormously. The maximum height of 3780 m exists at 08:00 BT while the minimum height of 850 m exists at 10:00 BT. The potential temperature increases with time and the accumulated energy lasts for 6 h. The specific humidity is greater than that in sunny days. The wind speed in boundary layer is larger than that of the sunny days, varying little with altitude and the wind direction is always dominated by west wind. There is deep cloud system which provides moisture. The ascending motion in clouds and descending motion under the clouds are main dynamic mechanism to promote graupel.