An unrestrained timber beam and a dovetail joint timber beam are placed in Lasa，and non-load test and step loading test are carried out，respectively，to study the strain parallel to grain of the timber beam under environmental temperature effect. According to the test data，the elasticity modulus parallel to grain of the wood，the tension and compression stiffness and rotation stiffness of the dovetail joints are identified. Considering the environmental temperature effect，the parameters are determined as variables related to the temperature variations. According to the construction features of the dovetail joint，the tension and compression stiffness in the temperature increasing stage and temperature decreasing stage are different. The influences of upper load and boundary conditions on the strain parallel to grain are analyzed. The results show that the strain variations of unrestrained beam under environmental temperature effect is unconcerned with the top load，and the strain variations of restrained beam increase along with the environmental temperature. The extra strain is induced by the second order moment of the counterforce at the beam end，which increases along with the temperature and the upper load. Finally，according to the theoretical formula of extra strain，the measured value and calculated value matched well，which indicates that the theoretical formula of the extra strain is rational.