〔Abstract〕 Objective To explore the biomechanical characteristics of short implants with different diameters in mandibular molars with severe jaw absorption, and to provide theoretical basis for the application of short implants in mandibular molar area. Methods CBCT data of patient were imported into Mimics and 3-Matic, and the finite element model of mandibular second molar repaired with short implant with diameter of 5.5 mm or 6.5 mm and length of 7 mm was established. The data were imported into three-dimensional finite element analysis software Marc Mentat, and high or low density bone was set up. An axial or buccal load of 150 N was applied to simulate clinical situation. A total of 8 models were established. Cortical bone stress, cancellous bone strain and implant displacement were calculated and analyzed. Results The cortical bone stress was concentrated in the neck of implant, and the cancellous bone strain was mainly distributed in the apical area of implant. The maximum values of cortical bone stress, cancellous bone strain and implant displacement were higher in buccal loading model than those in axial loading model, and they were higher in low density bone model than those in high density bone model under the same conditions. When the 5.5 mm diameter implant was loaded in buccal direction and the surrounding bone density was low, the maximum strain of cancellous bone was greater than the upper limit of physiological absorption of bone tissue 3 000 μstrain. Conclusion The application of short implants in mandibular molars with insufficient vertical bone mass is a feasible restoration scheme. But for patients with low bone mineral density, large diameter short implants can be appropriately selected, and the occlusal area and the cusp inclination can be appropriately reduced, so as to reduce the lateral force and obtain the stability and success of the implant in the short and long term.