A nanostructured complex, graphene oxide (G-COOH)/polystyrene (PS) nanofiber, was fabricated as a novel sensing material coated on a quartz crystal microbalance (QCM) to realize ammonia detection in this study. Nanoporous G-COOH/PS nanofibers with an average diameter of 569 nm were fabricated via electrospinning; the fibers were composed of ultrathin nanowires with a primary diameter of 37 nm. This unique structure presented a large surface active region, making the fibers an optimal candidate for gas-sensing applications. The physical and chemical properties of the G-COOH/PS nanofibers were characterized by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), N-2 physical adsorption, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The results indicated that the G-COOH/PS composite nanofibers had a mesoporous structure and that G-COOH sheets were randomly dispersed in the nanofibers. A gas-sensing test showed that the G-COOH/PS nanofibers, when incorporated into a QCM sensor system, exhibited good and desirable sensing behavior, including high sensitivity, fast response and good reversibility, making them a promising candidate as an ammonia detector.