The reasons for the total decarbonization failure of 30CrMnSiNi2A cold drawing hexagonal steel are analyzed， the results show that when the deformation amount of cold drawing is in the critical deformation range， the grains in the cold deformation area are coarse after annealing at 690 ℃， which affects the determination of total decarbonization. The recrystallized grains in the cold deformation zone area are refined after heat treatment when the single drawing deformation is between 21% and 30%. Therefore the accuracy of total decarbonization determination is eliminated because the deformation is in the critical deformation interval and the grain is coarse after drawing annealing.Through the production practice， the single deformation of cold drawing is controlled in the range of 20%-25%， and reduce annealing temperature to 630 ℃， the total decarbonization of S22 mm cold drawing rolled reaches the standard requirements， and the decarbonization improvement effect is obvious.