Ethylene is the most common ripening phytohormone in fruits, and excess ethylene can overripen the fruit. However, the in-field detection of ethylene is still limited. In this work, ultrafine Pd nanoparticles-decorated porous ZnO nanosheets (UPNP ZnO nanosheets) were conveniently synthesized through a facile solvent reduction method. The UPNP ZnO nanosheets were characterized using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrum, X-ray diffraction and X-ray photoelectron spectroscopy. The ZnO nanosheets were uniformly coated with Pd nanoparticles. The size of the Pd nanoparticle was very small, with a diameter of approximately 2 nm. Due to the unique structure of the porous ZnO nanosheets and the excellent catalytic properties of the ultrafine Pd nanoparticles, the as-prepared samples showed very high sensing performance in ethylene detection. The lowest detection concentration was 10 ppb, which is the lowest detection limit to our knowledge. It has been proved that the decoration of ultrafine Pd nanoparticles can largely increase the relative percentage of chemisorbed oxygen and deficient oxygen, which are benefits for ethylene oxidation, and actually accelerate the process of the sensing reaction. Furthermore, the UPNP ZnO nanosheets can even be applied in fruit maturity detection. Using mangos as an example, our experiment revealed that the response of UPNP ZnO nanosheets to mangos at different maturity stages was quite different. This result suggests that our product has broad application prospects in monitoring fruit ripening stage.