Potassium ions (K⁺) play an important role in biological organisms. Therefore, efficient and sensitive detection of K⁺ in biological complex samples is crucial for disease diagnosis. Herein, we propose a novel sensor based on crown ether (4-aminobenzo-18-crown-6, ABC) modified Au NPs for rapid colorimetric detection of K⁺. Recognition of K⁺ and constitution of the aggregates are posed by a sandwich complex of 2:1 between ABC moiety and K⁺. Based on the aggregation mechanism, this colorimetric detection method has superior selectivity and anti-interference ability for potassium ions (K⁺) compared with other ions. There is a good linear relationship (R² = 0.9979) in the range of 0–200 μM K⁺ concentration and this detection method can be successfully applied in the detection of K⁺ in real urine samples.