The change of filament size in fused deposition modeling (FDM) has an important influence on the printing precision. In this paper, the diameter changes of PLA single filament during FDM process from nozzle extrusion to deposition solidification were studied by experimental and numerical simulation methods. According to the characteristics of FDM process, the effects of process parameters (nozzle temperature, extrusion speed, printing speed, layer height) on the diameter of filament near the nozzle, extrusion expansion stage, stretching stage and deposition stage were quantitatively analyzed. The results show that increasing the nozzle temperature, increasing the extrusion speed and reducing the layer height are very helpful to adjust the dimensional stability of filament during printing process and improve the precision of the printed parts. Finally, by using the diameter formula during deposition and solidification, the "precision experiment" of filament diameter compensation is done, and the dimensional accuracy of printed products is improved by 74.1 % after compensation.