Firstly, four groups of timber beams with shrinkage cracks in diversified depth were made and one-third loading method to observe the experimental data and record the ultimate bearing capacity was carried out. Then, according to the experimental data, combining finite element stress analysis, and the model of bilinear constitutive model of wood, the critical indicators which indicated that bending failure become shear failure and the corresponding ultimate bearing capacity formula were deduced. After that, the bearing capacity formula was simplified as the bearing capacity reduction coefficient curve. Finally, by comparing theoretical analysis with experiment, the results show that the theoretical values of ultimate bearing capacity coincide well with the test values, and the test phenomenon is consistent with the judgement of the critical indicator.