Anthocyanins, natural dyes extracted from purple corn were added in the mixture of potato starch and agar to prepare a pH bio–indicator film. The UV–vis spectrum of anthocyanins extracted from purple corn shows different colors when the pH changes (pH 1.0–10.0). The Fourier transform infrared spectra (FTIR) of potato starch, agar and pH bio–indicator film did not show any significant difference due to a small amount of anthocyanins. However, the pH bio–indicator film changed the color after immersion in pH buffer solution from red to green when pH value changed from 7 to 10. The application of pH bio–indicator film was developed to inform spoilage of pork. The color of bio–indicator film change to green color within 24 h after stored fresh pork. The inform spoilage of pork could be clearly detected by the pH bio–indicator.