A novel, ultrasensitive, selective and flexible sensor strip based on polyaniline/polyamide-6 (PANI/PA-6) nano-fiber/net (NFN) membranes for naked-eye colorimetric detection of Cu²⁺ ions in water is successfully prepared by a facile electro-spinning/netting (ESN) process. The sensing mechanism involves the transformations between different oxidation and doping forms of PANI. Upon exposure to Cu²⁺ aqueous solution, the sensors exhibit two significant reflectance intensity decreasing bands at 435 and 650 nm which induce the color changes from white to blue dramatically. This new sensor shows colorimetric response specifically to Cu²⁺ ions (white-to-blue color change) over other possible interfering metal cations and allows for detection of Cu²⁺ in aqueous solution with a low detection limit of 1 ppb observing by naked eye. Additionally, the colorimetric responses are visualized quantitative by using a color-differentiation map prepared from converted RGB (red, green and blue) values. Furthermore, the as-prepared PANI/PA-6 NFN sensor strips could successfully combine with the color map, which suggested a promising analytical method as an economical alternative to traditional Cu²⁺ sensors and also provided a new insight into the design and development of a novel colorimetric sensing system based on the NFN platform.