The aging problem of composite insulator is more prominent with the improvement of voltage level and deterioration of the operating environments. Fine estimation of composite insulators can help in understanding the aging state accurately and providing guidance for subsequent treatment. In this paper, the sampling method for the fine estimation of field composite insulators' aging state is systemically studied based on Fourier Transform Infrared Spectroscopy (FTIR). Since determining the reference sample is a prerequisite for the fine estimation, the tests were conducted on the specimens with different depths from the upper surface of the shed. Next, the composite insulators from different manufacturers, different operating years and different operating environments were compared in order to explore the influence on sampling schemes. Furthermore, the sheds in different positions and different orientations were also analyzed to study the influencing rules. Finally, a set of sampling methods suitable for fine estimation of the field composite insulators' aging state were proposed, which could provide some references for the application of composite insulators in ultra-high voltage (UHV) projects and the formulation of related standards.