Fly ash from the incineration of municipal sewage sludge (MSS) contains high phosphorus content and can be a source of phosphorus resource. However, the utilization is limited due to the enrichment of trace metals in the fly ash. Co-firing of MSS and biomass was proposed to reduce the leaching toxicity of trace metals in the fly ash. The present study studies two typical trace metal's; arsenic (As) and zinc (Zn), present in the co-firing of MSS and biomass in a bubbling fluidized bed. The influence of biomass share (the relative amount of biomass in the mixture of biomass and MSS), biomass species, and combustion temperatures on the concentrations, speciation, and the leaching toxicity of Zn and As in fly ash was investigated. The results show that Zn is more stable after co-firing of MSS and biomass. The stability of Zn species in, the fly ash was the best at the incineration temperature of 1123 K The leaching concentration of Zn in the fly ash decreased, from a maximum of 22.53 mg/L to the minimum of 11.37 mg/L, as the amount of biomass increased. For As, the presence of minerals such as Ca and Fe in: biomass can provide chemical adsorption of As and promote the capture of As by forming arsenates. The stability of As species in the fly ash was better at the lower incineration temperature. Wheat straw and cotton stalk have the same influence on the leaching concentration of As in the fly ash. Co-firing MSS and biomass can effectively reduce the leaching toxicity of Zn in the fly ash, bin the leaching toxicity of As decreased only at 50% biomass.