When natural radionuclides are well distributed in materials, there is a fixed interrelationship between activities in any part of the material and the surface flux of alpha and beta rays. Placing an alpha detector on the surface can estimate radionuclide activities in the materials without damaging samples. This paper studied the relationship between gamma ray specific activities of natural radionuclides and alpha ray surface flux of decorative building materials. The results show that when the surface alpha ray activities of granite are less than 0.0052 Bq cm−2, or the surface alpha ray activities of polished tiles are less than 0.0082 Bq cm−2, their internal and external exposure indexes are in the range of “A” limits of national criteria of China, Limit of radionuclides in building materials. Because there is a great difference between the radioactive levels of the surface glaze and the matrix of color-glazed tiles, the natural radionuclides are not well distributed in color-glazed tiles, and the ratio between the alpha surface activity and the gamma ray activity in unit mass is not fixed. The surface alpha ray flux of materials cannot be used to estimate the internal and external exposure indexes of color-glazed tiles.