Water plays all essential role in determining the mechanical properties of silks and other structural proteins. Here we study the effects of moisture oil the elastic modulus of Antheraea pernyi silks, which exhibit all abrupt drop over a narrow range of relative humidity in the same way as synthetic polymers do in it temperature scan through their glass transition. A linear relationship between relative humidity and the transition temperature allows a simple transformation of the modulus-humidity curves into a general modulus-temperature relation for Antheraea pernyi silks. A model is presented to explain these observations quantitatively at the molecular level.