A novel method for description of the strain field distributions during the steel forging was proposed in the present study, i.e., function representation for the strain field. Through deduction and calculation of the strain field function, the deformation characteristic of the void defects was obtained and the empirical features of void closure were quantitatively calculated based on the method. The analytical results of the deformation degree in size of the void and the critical condition of void closure were verified by numerical simulation, which shows the deviation is within 7%. During the practical process, the void closing region determined by the method can be developed to prediction of internal void closure, which simplifies the models with void. This methodology provides the scientific basis for process design in consideration of elimination of internal void defects.