In order to detect the characteristics of refractive index of the oil mixed kerosene and diesel, the sensitive features of refractive index of long period fiber grating were analyzed by using the coupled-mode theory, the relationship between the content of diesel oil and the long period fiber grating’s resonant wavelength and transmitted intensity was obtained. The result shows that when the diesel oil of the mixed oil increases, the long period fiber grating resonance wavelength blue-shifts. And when the concentration of diesel oil reaches a certain concentration that the refractive index of the solution is equal to the cladding refractive index, the resonance wavelength will shift to the maximum. As the concentration of diesel changes 1%, the average shifts of resonance wavelength reaches 0.622 7 nm. As the diesel oil increases sequentially, the refractive index of mixed oil exceeds the cladding refractive index, and the resonant wavelength does not change basically, the shape changes greatly with the bandwidth decreasing and the peak loss increasing. And with the concentration of diesel oil increasing unceasingly, the peak loss increase and bandwidth diminishes. As the concentration of diesel changes 1%, the peak loss reaches 0.154 6 dB. So the long period fiber grating has a potential application prospect in the detection of mixed fuel concentration.