The under-filling defect of I-beam in a heavy truck front axle which occurs in the process debugging of the precision roll-forging and die-forging is studied. Finite element models of precision roll-forging and die-forging are established based on FEM software and the previous productive results. Then, the mechanism of the under-filing defect in I-beam is studied. And the effects of the three variables including width B, radius R and slope A of I-beam in the third step on the forging quality are investigated. In addition, the roll-forging die constructions for I-beam in previous three steps are redesigned and manufactured. The productive results show that I-beam can be fully filled, the debugging cycle is reduced significantly and the products quality is guaranteed.