The serious K-induced ash problems during biomass thermal conversion are closely related to release and transformation behaviors of K. The aim of this work is to explore the contribution of different occurrence forms of K on K release and transformation pathways among various K species during devolatilization of agricultural straw. Devolatilization experiments under different heat treatment conditions (i.e., temperature, heating rate, and atmosphere) were carried out using a fixed-bed apparatus, and occurrence forms of K in the virgin sample and residual chars were quantitatively determined by sequential chemical fraction analysis combined with inductively coupled plasma optical emission spectrometry. Unstable watersoluble K and ion-exchangeable K in wheat straw accounted for about 89.3% of total K and were mainly responsible for the K release. Part of unstable K was directly decomposed into atomic K or sublimated, and a large fraction was first bound to the char matrix to form char K and then transformed into other forms or released depending upon specific conditions. The release and transformation of K were promoted under intensive conditions (i.e., high temperature and heating rate) and were strongly dependent upon atmospheres. In comparison to N-2 conditions, a CO2 atmosphere inhibited K release and facilitated the formation of HCl-soluble K, while an O-2 atmosphere promoted K release and enhanced the formation of insoluble K. Finally, the functional mechanism for K release and transformation among various K species during volatilization was proposed.