A constitutive relation model under off-axis transverse compression is proposed for ancient wood， which has few parameters， simple physical meaning and easy access to parameters. The constitutive relation model introduces the Sun-Chen constitutive relation model in the field of unidirectional fiber composites， and combines the off-axis transverse compression characteristics of ancient wood to reflect the cumulative damage of ancient wood by a simple method， so as to accurately describe the mechanical response of ancient wood under off-axis transverse compression. Taking the replacement parts of the Forbidden City as samples， seven groups of 70 specimens are made. The corresponding stress-strain curve is obtained through the transverse compression test of specimens. The model parameters are solved by using the test results with off-axis angles of 0°， 45° and 90°， respectively， and the determination coefficient of the main curve is 0.94. Using this model， the yield strength and stress-strain curves with off-axis angles of 15°， 30°， 60° and 75°， respectively， are obtained. The comparison between the model calculation and testing results shows that using a small number of test results， the constitutive relationship model proposed in this paper can accurately calculate the transverse compressive yield strength and stress-strain curve of ancient wood with multiple off-axis angles. The constitutive relationship model can describe the characteristics of Dougong material compression damage， and it is also helpful in the protection of ancient wooden structures.