Arsenic (As) emission during sewage sludge incineration is harmful to the ecology and environment. Adding biomass to sewage sludge was proposed to facilitate the conversion of As3+ to As5+ with low toxicity during combustion. In this study, co-firing tests of two sewage sludge samples and four biomass samples were carried out to determine the migration and transformation of arsenic. The results show that the addition of biomass to municipal sewage sludge (MSS) facilitates the fixation of As in bottom ash in the combustion process. During combustion of MSS with biomass, mineral compounds in biomass, such as CaO, Ca2SiO4, or CaSO4, can afford additional reactive sites for the oxidation of arsenic by producing multifarious arsenates, and the availability of mineral active sites in cotton stalk (CTS) and corn stalk (CRS) is higher than that in wood and wheat straw (WS). The inherent mineral compounds in industrial sewage sludge (ISS), such as CaO and Ca2SiO4, provide sufficient reactive sites for the complete oxidation of arsenic to As5+ during ISS combustion. The addition of CTS stimulates the migration of arsenic vapor into bottom ash and the stabilization of arsenic into aluminosilicates during ISS combustion.