HFO-1234yf/iso-butane mixture is a promising working fluid of organic Rankine cycle, but the working fluid will be pyrolyzed under the high temperature heat source. In this study, the pyrolysis mechanism of HFO-1234yf/iso-butane mixture was studied by using density functional theory method and ReaxFF reactive force field. The initial pyrolysis reactions of HFO-1234yf and iso-butane, the pyrolysis process, the effects of HFO-1234yf to iso-butane ratio and temperature on the pyrolysis of HFO-1234yf/iso-butane mixture were investigated. The results indicated that the iso-butane of the HFO-1234yf/iso-butane mixture inhibited the pyrolysis of HFO-1234yf, the thermal stability of HFO-1234yf/iso-butane mixture was better than that of pure HFO-1234yf. Therefore, HFO-1234yf/iso-butane mixture has better thermal stability than HFO-1234yf and can match higher temperature heat source.