The influence of municipal sewage sludge addition on the agglomeration characteristic during straw combustion in a fluidized bed was investigated in a bubbling fluidized-bed reactor (5 kWth) with quartz sand as the bed material. In addition, chemical reactions between the alkali metal and three kinds of phosphorus-containing model compounds were studied to investigate the effect of phosphorus on the behavior of the alkali metal during wheat straw combustion. The element contents, the crystalline phases, the morphology, and the elemental composition in the residual ash samples from the combustion experiments were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy (SEM), respectively. The results show that the agglomeration tendency decreases and the defluidization time is prolonged during co-combustion of municipal sewage sludge and wheat straw. The addition of municipal sewage sludge in wheat straw leads to the formation of alkali-rich phosphate silicate that restrains the reaction between the alkali and quartz sand. XRD results exhibit that, besides aluminum, phosphorus in the municipal sewage sludge can react with the potassium in both fly ash and bottom ash to form compounds with a high melting temperature. Reactions between the phosphorus-rich additive and alkali metal in wheat straw lead to the formation of K-Ca-P compounds. The formation of alkali phosphates reveals the capturing mechanism of the potassium phosphate during combustion of the blend of municipal sewage sludge and wheat straw.