By using a novel multispectral imaging technology, the 2-D distributions of flame temperature and emissivity were measured in a 16 MW incinerator to co-fire municipal solid waste (MSW) and municipal sludge. A way to establish the relationship between the multispectral flame images and the temperature was proposed by combing the Newton iteration method and Hottel emissivity model. The results showed that the measured temperatures at different locations varied by 31.25% with a fixed steam evaporation rate, and 11.76% with different steam evaporation rates at a given port. The temperatures and emissivities decreased at upper locations due to the lower local soot particle concentration and the change of the measured flame temperatures with load were correlated with the MSW caloric values. Flame temperatures near the left wall were higher than those near the right wall. This deviation was caused by the high moisture content of municipal sludge that inhibited combustion. The emissivities of flame near the right wall were lower than those near the left wall due to the low fixed carbon in municipal sludge. The normalized flame emissivities between the left and the right walls indicated that obvious differences existed in the radiative characteristics of soot, which confirmed the uneven mixing of MSW and municipal sludge. Besides, a spectrometer system was used to measure the release of alkali metal elements including Na, K during the incineration process. The characteristic spectra showed that the alkali metal radiative intensity was related to the moisture content in the wastes. Overall, these results justified that the multi-wavelength thermometry was feasible for monitoring combustion in the MSW incinerator.