In the present study, cogasification of straw and polyethylene was investigated in a fixed-bed gasification system. The fusion characteristics of ash during cogasification of two different kind of straw and polyethylene were investigated by AFT (Ash Fusion Test), XRD (X-ray diffraction), XRF (X-ray fluorescence), and SEM-EDX (scanning electron microscopy and energy dispersive X-ray). The results showed that the addition of polyethylene had little influence on the mineral species in the gasified ash and only affected the deformation temperature (DT) of gasified ash. The degree of disintegration of gasified ash was positively correlated to the amount of polyethylene, which was beneficial for the gasification process. The changes in relative contents of acidic oxides (SiO2+P2O5+Al2O3+SO3) and basic oxides (Na2O+K2O+CaO+MgO+Fe2O3) showed a consistent trend in influencing DT of the gasified ashes of two kinds of mixtures while showed different trends in influencing the softening temperatures (ST). PE addition reduces the amount of KCl formed in gasification of RS.