In this paper, the effect of fuel sensitivity (S) of 0 and 6 on the flame development and combustion luminescence spectrum in the engine cylinder are studied in an optical engine by using flame high-speed imaging technology and self-luminescence spectroscopy. The combustion model was transitioned from the new partially premixed combustion to diesel combustion mode by changing the injection timing (SOI= -25, -15, -5°CA ATDC), and the fuel sensitivity is changed by using n-heptane, iso-octane, ethanol mixed fuel. The results show that in the PPC model(-25°CA ATDC), the flame development process starts from the area near the wall and then develops toward the center of the combustion chamber and it is a similar flame propagation process, and a new unburned area is formed in the lower part of the combustion chamber. Sensitivity has a more significant impact on the combustion phase and less on the development history of the combustion flame in the cylinder. The high-sensitivity fuel suppresses when the OH and CH band spectra appear, and the spectral intensity is lower. With the change of fuel injection time, the spectral change trend of the two sensitive fuels is the same. The high-sensitivity fuel suppresses the high-temperature reaction process, weakens the soot radiation and reduces the spectral intensity. In the transition area between PPC and CDC (-15°CA ATDC), the combustion flame glows brighter than -25°CA ATDC, and the combustion reaction rate is faster than the reaction rate at -25°CA ATDC. The influence of high and low sensitivity fuels on the cylinder pressure heat release rate is similar to -25°CA ATDC. The combustion reaction is more intense, the heat release rate is high, and the soot appears earlier. The intensity of the spectrum at this time of fuel injection is higher than that in the PPC model, indicating that the CO oxidation reaction and soot radiation are stronger. In CDC mode, due to the low fuel activity used, the time of combustion heat release is too delayed, the heat release is small, and the combustion pressure in the cylinder is low, which is close to the misfire condition. Therefore, the effect of fuel sensitivity on cylinder pressure and heat release rate is not significant. The blue flame of low-sensitivity fuel first appears in the center of the combustion chamber at the beginning of the combustion, and the ignition flame appears earlier, and then the blue flame spreads from the center to the surroundings, showing a combustion process dominated by flame propagation. In the later stage of the combustion, the local mixed gas passes the dense zone causes the bright yellow flame area to increase and spread to the surroundings gradually. The flame development trend of high-sensitivity fuel is similar to that of low-sensitivity fuel, and the brightness and area of yellow flame are small. The appearance time of OH and CH band spectra of high-sensitivity and low-sensitivity fuels is similar, and the spectral intensity of high-sensitivity fuels is low. It may be because at the time of fuel injection, the flame retardation period is long enough, and the oxidation of ethanol in the susceptible fuel is the dominant factor. Comprehensive analysis shows that the flame development structure and spectrum development process are mainly affected by fuel injection time. The fuel sensitivity mainly affects the ignition time and flame self-luminescence spectrum intensity, and the spectrum intensity of low-sensitivity fuels is great.