We designed and synthesized an amphiphilic conjugated polymer P1 based on the 2,2′-biimidazole unit, and then investigated its sensing properties as an aggregation-based platform. P1 shows multicolor emission in organic solvents at different water contents, and is demonstrated to be the first multicolor fluorescent sensor for water. And it exhibits visually noticeable multicolor fluorescence response to Ag⁺ in aqueous dioxane and to Hg²⁺ ions in aqueous ethanol. In addition, the polymer–Hg²⁺ complex system can be used as an efficient cysteine fluorescent sensor based on the aggregation–disaggregation strategy. These results validate P1 as an efficient multifunctional platform with multicolor fluorescence response.