Based on the experimental and numerical simulation, the mechanism of micron dimension crack of crankshaft appeared in the area of inner parting plan of rod journal and transition fillet was analyzed in the view of the billet inner freedom defects’ transition during the forging. The billet was divided into two layers, named the outer higher quality layer and inner lower quality layer, and the corresponding finite element model of billet was built. The flow and distribution of two layers metals during forging were simulated by Deform 3D and the trend of accumulate of inner defects was achieved as well as the influence law of forging velocity on two layers’ flowing. The results showed that the inner low quality metal was distributed in the area of inner parting plan of rod journal and transition fillet and exposed on forging surface, which was the dangerous area. The metal on the cross section of rod journal flowing only at a direction intensified the trend of defect accumulating in the dangerous area. According to the experiment results, the micron dimension crack was a kind of hairline crack. The inclusion which contained MnS and MoS and transited into the surface of forging from inside of billet was extended along with the forming direction.