Chinese ancient wooden beams are easy to produce level cracks under loads. To protect Chinese ancient wooden buildings, crack extension characteristics of an ancient beam under level load were studied by XFEM technique. Based on properties of wood material as well as hypothesis that only level crack occurred on the beam, simplified XFEM (extended finite element method) of the ancient beam was built. Level load was applied to the model. Displacement, principle stress distributions of the model during course of crack extension were studied, and corresponding influence factors were also discussed. Results show that with the increase of load step, peak deformation value of the beam increases little, but its peak principle stress value increases obviously near end of the crack; the increase of load leads to obviously increase of peak value of principle stress of the beam, however the increase of initial length of crack affects little to mechanical performances of the beam. Besides, XFEM technique is proved to be effective for analysis of crack extension of Chinese ancient wooden buildings.