Abstract: Bombyx mori silk and spider dragline silk have many advantages such as excellent mechanical properties, biocompatibility and biodegradability. Compared with bombyx mori silk, spider dragline silk has more excellent comprehensive mechanical properties. However, spiders cannot be raised and spider silks cannot be launched into mass production. The mechanical difference between two kinds of fibers could be caused by the multi-level structure. This article describes the primary structure, secondary structure, β-crystal network (nanofibril) structure and the formation process of multi-level structure of the bombyx mori silk and spider dragline silk fibers. The effects of the hydrophobic and hydrophilic domain, molecular mass and crystal network structure on the mechanical properties of silk protein fibers are reviewed. Among them, the strength of the crystal network is the key to the mechanical properties, which is determined by four factors of topological structure, correlation length, degree of orientation, and strength of the connection or interaction. The summary of these four factors provides guidance for the control and improvement of the mechanical properties. Super strong silk fibers can be obtained by controlling the crystal network using regenerated silk fibroin protein or specific recombinant spider silk protein with genetic engineering technique as raw materials. Based on the concept of multi-level structure of bombyx mori silk and spider dragline silk, the crystal networks in particular, the new understanding acquired will transfer the research and engineering of protein fiber to a new phase.