Like most major ampullate silks of spider, the length of Antheraea pernyi silkworm silk can shrink to a certain degree when the fiber is in contact with water. However, what happens in terms of molecule chain level and how it correlates to the mechanical properties of the silk during its contraction is not yet fully understood. Here, we investigate the water induced mechanical property changes as well as the structure transition of two kinds of A. pernyi silk fiber, which are forcibly reeled from two different individuals (silkworm a and silkworm b; the silk fiber from either one represents the lower and upper limit of the distribution of mechanical properties, respectively). The tensile test results present that most of the mechanical parameters except the post-yield modulus and breaking strain for both silk fibers have the same variation trend before and after their water contraction. Synchrotron FTIR and Raman spectra show that the native filament from silkworm a contains more alpha-helix structures than that in silkworm b filament, and these alpha-helices are partially converted to beta-sheet structures after the contraction of the fibers, while the order of both beta-sheet and alpha-helix slightly increase. On the other side, the content and orientation of both secondary structural components in silkworm b fiber keep unchanged, no matter if it is native or contracted. C-13 CP/MAS NMR results further indicate that the alpha-helix/random coil to beta-sheet conformational transition that occurred in the silk of silkworm a corresponds the Ala residues. Based upon these results, the detailed structure transition models of both as-reeled A. pernyi silk fibers during water contraction are proposed finally to interpret their properties transformation.