Abstract: To measure accurately the thickness and its distribution of capsule, the white light reflection spectrum measurement technology is studied. The basic principle of white light reflection spectrum and the corresponding data processing methods, such as the extreme value method, the peak fitting method and the interference order correction method, are analyzed. Based on the white light reflection spectrum and the precision rotary axis system, the measuring apparatus applied to determine the shell thickness and its distribution of the capsule is developed. The white light reflectance spectrum measurement, data processing and reliability verification experiments of GDP capsule are carried out and the circular thickness distribution curve of capsule is obtained. The research results show that the thickness and its distribution of capsule can be measured accurately by white light reflectance spectrum technology based on spectral peak fitting method and interference order correction method and the measurement error is less than 0.1 μm.