Abstract: The convective boundary layer (CBL) in the arid zone of northwest China can reach heights of 4 km in summer and 800 m in winter according to the observation. Its cause was quantitatively studied using the thermodynamic method. The results show that: (1) the averaged CBL height can be accounted in terms of sensible heat for its 51.5% in summer and 61.4% in winter with the percentages by sensible heat up to 63.4% in summer and 73.1% in winter when the maxima of CBL heights reached, and the degree of contribution of sensible heat to CBL in winter bigger distinctly than that in summer; (2) the contributions of entrainment caused by buoyancy are 18.1% in summer and 9.4% in winter, respectively; (3) the coupled effects of sensible heat and buoyancy entrainment are as high as 69.6% in summer and 70.8% in winter as applied to averaged CBL heights, but even as higher as 81% in summer and 84.6% in winter as to their maxima. In conclusion, thermal effects including sensible heat and buoyancy entrainment together are crucial to forming such high CBL height over the arid area, in which sensible heat is more important, in some sense, in consistent with the general knowledge.