Proposed in this paper is a surface-potential-based drain current model for organic thin-film transistor ( OTFT) that takes into consideration the exponential distribution of the trap state density within the bandgaps. In the process of modeling,the diffusion and drift currents are approximately identified by means of the charge sheet approach. Then,a high-precision analytical solution for surface potential is presented via Taylor expansion. Moreover,the transfer and temperature characteristics of OTFT are described based on the variable-range hopping theory,i. e.,the thermally-activated tunneling mechanism of carriers between localized states. It is found that the calculation results accord well with the experimental ones,which shows that the proposed model is applicable to the circuit simulator.