In this paper, experimental and numerical simulation studies on the forming limit diagram of single-point incremental forming (SPIF) were described. We proposed a novel method for evaluating the forming limit in incremental forming. The proposed method utilizes the forming limit angle and the maximum thinning rate. The effects of the forming and processing parameters on the forming limit of an aluminum sheet during incremental forming were investigated using a combination of simulation analysis and experimental verification. The obtained results show that the forming limit is large for 1060Al and 6061Al when the initial thickness is 1.5 mm for a single parameter change for SPIF. For 1060Al, the step size is in the range of 0.8–1.5 mm. When the step size is 0.8 mm, the incremental forming limit is large and the forming precision is high. For 1060Al and 6061Al, the incremental forming limit is at the maximum level when the tool radius is 6.0 mm, and when the tool radius is 5.0 mm, the best forming precision can be achieved. It is of great theoretical significance and practical engineering value to study the effects of various forming parameters on the forming limit of metal sheet SPIF.