Abstract: The mechanical property degradation of timber is affected by many factors. However, there are relatively few studies on the degradation of compressive strength of the material under long-term loadings. Therefore, mechanical tests and creep simulation on the replacement beam from a typical Tibetan ancient timber structure were conducted, and the degradation rate of compressive strength in three main directions in combination with the inherent differences of heartwood and of sapwood were quantitatively analyzed. Research results showed that 44.44% of the control group had significant differences of compressive strength from different parts of the beam under 5% significance level; that the creep simulation results of the 300-year service life beam showed that the compression and tensile stresses at the top and bottom of the midspan are relatively large, while the degradation rates of the longitudinal compressive strength at the top of the midspan and, of the tangential compressive strength at the bottom of the end are relatively large, which are 4.7% and 3.9% respectively; that for the wood with regular texture, the theoretical degraded compressive strength is close to the experimental value when only considering the load history and the difference of heartwood and sapwood, while the difference is large when the wood has obvious pith eccentricity; and that compared with the along grain compressive strength, the pith eccentricity has a greater impact on the transverse compressive strength.