Abstract: The varied-level fibril packing models in the multiply layers of a cotton fiber were widely concerned previously, and different varied-level fibril models were brought forward. This paper mainly concerned on the fractal characteristics of secondary wall S2 of a cotton fiber, assumed that S2 is composed of three grade fractal structures with the four level fibrils respectively, and adopted three fibril packing models, i.e.—round, intermix and elliptic styles. Their radius, fractal dimensions and crystallinity were calculated and compared, and then, modified the hypotheses. Models have been modified and examined according to the error. The results showed that fractal dimensions of the three different level fibril structures are 1.6575, 1.647 and 1.732 respectively, the crystallinity of improved fibril bundle is 64.7% and closed to its measured result 66.7% of cotton fiber.