The fine-blanking procedure for AISI-1045 and AISI-1020 under the condition of various blanking allowance has been simulated by the DEFORM-2D software, and a large number of fine-blanking experiments have been made. Some key factors, like blanking allowance affecting upon the ejector force, the perfect ratio of fracture and the fracture procedure, are discussed, and the principles for choosing value of blanking allowance are given. In order to reduce ejector force, increase the perfect ratio of fracture and separate scrap from sheet metal easily, a smaller value C of blanking allowance should be adopted whenever possible. The value C of blanking allowance should be chosen according to the mechanism capability. For some types of steels which have good plasticity, a larger value C of blanking allowance should be chosen, so that the work-piece without blanking edge could be obtained. For some types of steels which have poor plasticity, a smaller value C of blanking allowance should be chosen to obtain blanking work-piece with higher quality. Generally speaking, the optimized value of blanking allowance should be from 0.15mm to 0.35mm, and the largest value must be less than 0.5mm.The research result has instructive significance and reference value to improve the quality of work pieces and the perfect ratio of sheared band.