Through the two-year long-term MicroWave Radiometer (MWR) observations at the Beijing National Atmospheric Observation Test Base, the Atmospheric Boundary Layer (ABL) height in this area is retrieved by the potential temperature parcel method. The diurnal and monthly changes of the ABL height are analyzed. Moreover, the ABL heights retrieved by the MWR and radiosonde sounding are compared. The characteristics of the change in the ABL height during the day have a good relationship with the change of sunshine duration. Besides the thermal effect, the vertical wind velocity influences the ABL height. Stronger winds near the surface play an essential role in raising the ABL height, leading to the height of the nocturnal ABL from January to April being significantly higher than that in other months. Due to good daytime atmospheric diffusion conditions in February, March, April, September, and October, the maximum monthly mean ABL height exceeded 3 km in all these months. The maximum monthly mean daytime ABL height was 1461 m, and the minimum monthly mean nocturnal ABL height was 242 m. The mean monthly ABL height in February was the highest in a year, 943 m, and the lowest mean monthly ABL height was in December with a height of 426 m. Since the weather was mostly rainy and cloudy in July and August, the mean monthly ABL height was low in these two months. The inversion results of the MWR data was in good consonance with that of the sounding data. The corresponding Pearson’s correlation coefficient reached 0.874, and the root mean square error was only 300m.