Experiments on combustion of oil shale sludge and semicoke were conducted on TG-FTIR to investigate the mechanism of co-combustion involved. The synergy effect of sludge and semicoke was estimated by using the interaction coefficient and the relative error of mean square root. In addition, the activation energy was calculated by means of Coats Redfern, DAEM, FWO, and Starink. The results show that the whole process contains three stages: light components combustion (130-350 degrees C), heavy components and volatiles co-combustion (350-650 degrees C), and fixed carbon combustion (650-750 degrees C). During the combustion, synergy behaves differently with variation of mixing proportion in mixture and heating rates. DTG curves of mixtures can be decomposed into five Gaussian fitting peaks, respectively corresponding to five independent parallel reactions. From the results of Coats Redfern, one can see that the kinetic triplets are related to mixture proportion, combustion stage, and heating rate. In addtion, the results of DAEM, FWO, and Starink indicate the three above are fit for calculating kinetic parameters of mixture and also suggest that combustion of mixture is a multiple reaction. It can be noted that there is a positive relationship between release rate of flue gas and proportion of sludge. Besides, the nonlinearity between A(CO)/A(CO2) and the proportion of sludge indicates that an overlarge proportion of sludge is disadvantageous for sufficient burning. According to the combustion characteristics, the kinetic analysis, synergy, and FTIR analysis of flue gas, the optimal blending ratio is 20%/80% of sludge/semicoke.