A one-pot "green'' synthesis of water soluble and pH-responsive natural silk fibroin (SF)-stabilized fluorescent copper nanoclusters (CuNCs) was reported without using any additional reducing agents, and the as-prepared SF@CuNCs exhibited a fluorescence emission at 420 nm with highly stable properties. The reversible pH sensor of the fluorescent SF@CuNCs has been designed based on the obvious fluorescence enhancing properties of CuNCs. Significantly, the SF@CuNCs described here were employed as pH sensors by virtue of the fluorescence intensity of their sensitive response to fluctuating pH in a linear range of 6.08-10.05. Meanwhile, the fluorescence intensity of the SF@CuNCs increased by around 8-fold at high pH values compared with a low pH of 6.08. As expected, the pH sensor was sensitive to different buffer solutions, and also presented a good linear relationship in different buffer solutions. Besides, the ionic strength of the buffers had a slight influence on the pH-responsive behavior and various metal ions showed almost no effects. The proposed method was successfully used to detect the pH of real water samples, thereby indicating its potential practical application.