This paper presents a new approach to estimate the Arrhenius parameters as well as the reaction model function of cellulose pyrolysis reaction. Reduced time plot (RTP) was employed to choose a proper form of reaction model function for cellulose pyrolysis reaction. A state-of-the-art thermobalance (TB) that is able to form real isothermal reaction conditions was introduced to construct RTPs from isothermal decomposition kinetic data. The reaction model function of cellulose pyrolysis reaction would be accounted for by Avrami-Erofeev function, n(1-α){-ln(1-a)}1-1/n where n is determined to be 3.69.