With the rapid development of point-of-care (POC) technologies, development of sensitive method featured with fast analysis and affordable devices has become an emerging requirement for practical applications. In this study, we introduced a smartphone-based RGB analysis system for the sensitive detection of acid phosphatase (ACP) enzyme activity. In the presence of ACP, l-ascorbic acid 2-phosphate (AAP) can be converted into ascorbic acid (AA), which can reduce Ag+ to Ag0 and format the Au@Ag core-shell nanostructure. This morphology change of the Au@Ag core-shell would trigger a significant color variation (pink to yellow). A good linear relationship between the RGB model parameter and the concentration of ACP could be obtained with a detection limit of 0.1 U L-1. Moreover, this sensing strategy is suitable for the detection of ACP in practical serum samples. Thus, this simple but powerful protocol has great potential application for on-site detection of ACP in future complex biological samples.