Five biomass including cotton stalk (CS), sunflower stalk (SS), wheat stalk (WS), rice husk (RH) and maize stalk (MS) were pyrolyzed in an entrained flow reactor under reburning condition. The chlorine release fraction was determined based on the analysis of each biomass and the relevant bio-coke measured by digital ion meter. The effects of biomass species, reaction temperature (T), residence time (τ), stoichiometric ratio (SR2), and initial oxygen content in the simulated flue gas on chlorine release were analyzed. The obtained results indicated that the chlorine release fraction increases with the increasing of reaction temperature, and all biomass have a higher chlorine release fraction of 94.6%–100% at high reaction temperature. Stoichiometric ratio has little influence on chlorine release. The chlorine release fraction shows a significant increase from 80.3% to 97.1% with increasing initial oxygen content in the simulated flue gas from 0% to 4%.