The utility model discloses a fuel composition sensor. In one embodiment, the fuel composition sensor comprises a fuel inlet, a sensor module and a fuel outlet which are connected in series in a fuel supply pipe of an engine. In the other embodiment, the fuel composition sensor comprises a sensor module arranged in a fuel pump bracket. In the two embodiments, the sensor module at least comprises a dielectric constant and electric conductivity sensing element for acquiring the dielectric constant and electric conductivity of a fuel simultaneously, a frequency generator for sending a pulse enable signal to the dielectric constant and electric conductivity sensing element, a linearization processing module for converting the dielectric constant and electric conductivity of the fuel into a fuel composition proportion, and a signal output end for outputting the fuel composition proportion to an engine control system. The fuel composition sensor can accurately judge the fuel compositions of the fuel which is currently used by the engine so as to provide reliable bases for the fuel injection quantity and ignition timing of the engine.