It is common to cause errors in the selection of building foundation in karst terrain. The root reason is that the practical difficulty of karst treatment in foundation cannot be reasonably reflected by the existing evaluation of karst degree. Taking New Terminal of Nanning Wuxu International Airport as an engineering case, this article analyzes the limitation of the existing evaluation indices of karst degree, demonstrates the advanced rationality of intensity dissolution layer’s thickness as the evaluation index of karst degree, and proposes the recommended standard of the evaluation of karst degree. Results show that the intensity dissolution layer, divided by dissolution ratio distribution curve with depth in foundations, comprehensively considers the main depth range of dissolution degree of rock surface and the developing of cavern dissolution, and its distribution characteristics can reflect the most complex depth range of karst foundation. As the evaluation index of karst degree, it is reasonable. Its recommended values are suggested as follows, When the thickness of an intensity dissolution layer is less than 3.0 m, more than or equal to 3.0 m and less than 6.0 m and more than or equal to 6.0 m, the karst degree can be determined as weak development degree, medium development degree and strong development degree.