This study aims to investigate the influence of different neck structures on implant stress distribution. A computer-aided design (CAD) mandibular model based on adult mandibular computed tomography (CT) information was first established, and then used to create a mandibular model and a related mandibular implant model by using Solidworks. The implant was then fitted to the mandibular first molar. Four neck-type implants, i.e., uniform-threaded implant (UT), wide-neck implant (WN), implant with single-microthreaded neck (SMN), and implant with double-microthreaded neck (DMN), received axial 100-N, horizontal 50-N, and oblique 100-N loads, by using the Ansys Workbench software; the Von Mises equivalent stress (ES) and maximum principal stress (MPS) in different implant groups were then compared. Axial 100-N load: the maximum ES in the cortical bone area (CB-MES, MPa) was 46.58, 35.25, 20.98, and 58.01 in UT, WN, SMN, and DMN respectively; the MPS at the superior edge of cortical bone (CB-MPS, MPa) was 0.97, 0.89, 0.79, and 1.22 in the same four groups respectively. Horizontal 50-N load: CB-MES was 53.21, 41.93, 36.95, and 64.18 in UT, WN, SMN, and DMN respectively; CB-MPS was 41.77, 39.61, 29.67, and 46.99 in the same four groups respectively. Oblique 100-N load: CB-MES was 49.37, 47.78, 34.02, and 78.52 in UT, WN, SMN, and DMN respectively; CB-MPS was 29.35, 21.28, 15.34, and 34.69 in the same four groups respectively. CB-MES and CB-MPS of SMN were the smallest.