Objective To design two kinds of fixed full-arch implant-supported prosthesis and to investigate their stress distribution in the different vertical heights of the edentulous mandible. Methods The cone-beam computed tomography(CBCT) scan data of the upper denture and mandible were collected to establish the base models of the mandible. The vertical heights of the mandible models were lowered by 0, 5, 10 and 15 mm, and the three-dimensional finite element models of implants with different vertical heights supported by mental foramen 6 parallel implants and supported by "All-on-Four" were established, including 2 groups (8 models):6-implant group and 4-implant group. A 250 N vertical force was applied to the central fossa of the right mandibular first molar. The stress of each part of the models was analyzed by Ansys 15.0 software. Results Under the same loading condition, the maximum stress value of the implants was in the neck of the implants. The maximum stress distribution of 8 models showed that the maximum stress value was 40.12-49.06 MPa for the 4-implant group and was 80.62-109.64 MPa for the 6-implant group, with the latter being two folds that of the former. In two groups, the maximum stress was the lowest when the vertical height reduction was 5 mm. With the decrease of the mandible vertical height, the stress increased gradually. However, the maximum stress of the 0 mm-height-reduced models was larger than that of 5 mm-height-reduced models. Conclusion There is no destructive stress under certain extreme load in both 4-implant group and 6-implant group, with the 4-implant group having a more reasonable stress distribution. The 5 mm-hight-reduced models have the minimum stress, indicating that appropriate vertical height reduction can lead to more reasonable implant stress distribution.