Objective. To evaluate biomechanics of the new-designed cage for lumbar interbody fusion by using FEMs analysis.Methods. Five validated finite element models of L4–L5 lumbar segments were established to simulate the intact lumbar segement, lumbar discectomy and lumbar discectomy instrumented posterior lumbar interbody fusion with single three kinds of cage (threaded cage, impacted cage and new expanded cage), and analyzed the biomechanics of these models.Results. The maximum ranges of motion and the maximum stress of facet joint were appeared in discectomy model, and the insertion of cages obviously decreased the range and the maximum stresses in all motions. However, only the ranges in expanded cage model were lower than those in intact model. Conclusions. The new-designed expanded cage resulted in highest stability and lowest stress of facet joint in fusion models.