The stability of Au nanoparticles (AuNPs) is easily affected by environmental factors, and the use of expensive disposable AuNP-based probes not only wastes resources but also increases the cost of detection. Therefore, there is an urgent need to develop stable and recyclable AuNP-based probes. Here, highly stable AuNPs were synthesized via an in situ reduction reaction between HAuCl₄ and hyperbranched polyethylenimine (HPEI). We used HPEI to replace the small molecule reducing agent as a green reducer and template for the green and facile preparation of AuNPs as well as a good stabilizer and decorator of AuNPs. Due to the good properties (massive amount of amino groups, excellent reduction ability and hyperbranched topology structure) of HPEI, the HPEI–AuNPs are highly stable under high temperatures, strong ionic strength and different pH conditions. Meanwhile, the reductive HPEI was also used as the reducing agent to reduce Ag⁺ to Ag⁰; this can combine with AuNPs to form stable Au–Ag core–shell NPs accompanied by a special red-to-brown color change, achieving the colorimetric detection of Ag⁺ by the naked eye. Moreover, this HPEI–AuNP probe not only exhibits good selectivity and high sensitivity (a LOD of 0.41 μM) towards Ag⁺ but has also been successfully applied in real water samples (drinking and tap water). More importantly, by taking advantage of an etching reaction of the Ag shell of the Au–Ag core–shell NPs, a recoverable and reusable HPEI–AuNP probe towards Ag⁺ was developed successfully. This novel colorimetric system shows excellent recyclability and reusability, which is inspirational for the development of low-cost AuNP colorimetric probes and consistent with the idea of green chemistry.