A facile, ultrasensitive, and selective sensor strip utilizing 4-amino-3-penten-2-one (fluoral-p) functionalized electrospun polyacrylonitrile (PAN) (PAN/fluoral-p) nanofibrous membranes has been successfully developed for naked-eye colorimetric assay of formaldehyde. The sensor strips presented a significant reflectance decreasing band at 417 nm which induced a vivid color change from white to yellow and achieved a much lower naked-eye detection limit of 40 ppb compared with the World Health Organization standard (80 ppb). Based on the specific Hantzsch reaction between fluoral-p and formaldehyde, the as-prepared PAN/fluoral-p membranes are highly selective to formaldehyde with little interference from other volatile organic compounds and the proposed mechanism of this reaction is discussed carefully. Moreover, the colorimetric responses were visually quantitative using UV-vis spectra and the color difference calculated from L*, a*, b* values. Furthermore, due to the extremely large surface area and high porosity of the as-spun PAN nanofibrous membranes, the sensitivity of the nanofibrous membranes-based strips is much higher than traditional filter paper-based ones. Hence, such promising portable colorimetric sensor strips could not only potentially allow for assaying gaseous formaldehyde, but also facilitate the design and development of a novel colorimetric sensing system based on nanofibrous membranes.