Owing to the importance of geometrical design of clinching tools, the flat-clinching were investigated numerically and experimentally to determine the optimal geometrical parameters in this study. The geometrical parameters in flat-clinching, including punch corner radius, draft angle of the punch, blank holder corner radius, blank holder radius, and punch velocity, were optimized using the orthogonal experimental design simulation method based on the evaluation of interlocking. The simulation results show that the punch corner radius and the blank holder radius have a significant impact on the clinched joint, whereas the draft angle of the punch and blank holder corner radius have little influence on the clinched joint. The simulation results were validated through an experimental setup testing on material aluminum alloy Al5052. The experimental results have a good agreement with the orthogonal experimental design simulation.