X-ray diffraction (XRD) spectrum is an important method to analyze the clay mineral composition of the soil, but conventional XRD analysis has some problems such as complex preprocessing, long time-consuming, low spectral resolution and insufficient scanning. So based on the 4B9A diffraction experiment station of Beijing Synchrotron Radiation Facility (BSRF), 7 zonal forest soils (including brown coniferous forest, dark brown soil, brown soil, yellow brown soil, yellow soil, red soil and red brick soil) in different climate zones (temperate zone, subtropical zone and tropical zone) in the east of China were studied in this study. The characteristics of clay mineral composition in soils were analyzed by using synchrotron radiation X-ray diffraction (SR-XRD) spectra. The results showed that: (1) the texture of temperate soil (i. e. brown coniferous forest, dark brown soil and brown soil) was mainly clay loam, while that of subtropical soil (i.e. yellow brown soil, yellow soil and red soil) was distributed on the boundary line between clay and clay loam; from subtropical to tropical, the texture of soil presented the changing trend of clay loam and sand. (2) Sa of the soil showed the trend of temperate soil (5.22) > tropical soil (4.53) > subtropical soil (4.49); similarly, Saf also showed the trend of temperate soil (4.06) > tropical soil (3.53) > subtropical soil (3.36). (3) From the temperate zone to the tropical zone, the content of kaolinite and crystallinity of clay minerals in the soil increased, while the content of illite decreased gradually. In the temperate zone, the clay minerals were mainly 2∶1 type, while in the tropical zone, the clay minerals were mainly 1∶1 type. The existence of two kinds of clay minerals, i.e. illite mixed layer and feldspar, was identified in the SR-XRD spectrum, while was not identified in the conventional XRD spectrum. (4) Compared with the conventional XRD method, the SR-XRD method had the following advantages in the analysis of soil clay mineral composition: First, the preprocessing work was less, the conventional XRD method needed to determinate clay samples in different types of glass slides, while the SR-XRD method could directly determine clay samples; Second, the optical spectrum resolution was high, and the resolution of SR-XRD method could reach ≤100 nm, so more clay could be detected; Third, the optimized scanning step length of SR-XRD method could be 0.04, which was two times as that of conventional XRD method, thus improving the testing efficiency; Four, the scanning identification degree was increased, and the viewing time of SR-XRD method could be optimized to 4 minutes, which was 2 times of that of conventional XRD method, which made the scanning of samples more full.