Ascorbic acid (AA) is a vitamin similar to glucose polyol and is involved in various physiological and biochemical processes in the human body. Therefore, it is crucial to develop a quick and easy method for identifying AA to assess the freshness and nutritional value of fruits. In this study, we synthesized a novel Au@Pt nanoparticle structure, which exhibited excellent peroxide-like activity, by a one-step method. A colorimetric sensor for AA detection was then constructed on the basis of the chromogenic system of a nanoenzyme and 3,3’,5,5’-tetramethylbenzidine (TMB). This colorimetric sensor had a low detection limit of 0.213 μM and showed a good linear relationship in the concentration range of 5 to 345 μM. By integrating this sensor with a smartphone-connected visual sensing platform using test paper, we enabled a quick, portable, and accurate visual monitoring of AA in fruits. In this work, we not only introduced a smartphone-assisted colorimetric sensor and a new and effective noble metal nanozyme, but also proposed a creative design approach for developing an inexpensive, user-friendly, and highly sensitive vitamin detection instrument.