According to the construction and loading features of Tibetan ancient timber beam,a nonlinear spring was added at the end of the beam and the simplified mechanical model of temperature effect on the timber beam was established. Considering the second order of the counterforce of beam end,a theoretical model of the strain in the grain direction of the mid-span beam bottom under temperature effect was proposed. The strain in a temperature cycle was divided into four parts for calculations. A timber beam of a Tibetan ancient building from a monitoring system was employed as a calculation example. The results show that the strain returns to the initial value in a temperature cycle,while the curves of the temperature rising stage and temperature decreasing stage are not coincident but divided into two parts. The calculated strain curve matches well with the measurement. Finally,the parameter analysis of the structure parameters affecting the strain variations was proceed,and the results show that the different starting temperature makes no difference for the analysis results. Elasticity module is the most sensitive parameter,while the tension and compression stiffness of the dovetail joint is the least sensitive parameter.