The K&C material model, one of the widely used concrete models implemented in LS-DYNA for blast or impact analysis, is modified to improve its numerical accuracy for steel-fiber reinforced concrete (SFRC). Many input parameters are required in this model to describe the material properties of SFRC. To address this, an improved calibration method that can easily predict the stress–strain relationships of SFRC by modifying the three strength surfaces and the damage function η(λ) based on uniaxial and triaxial compression data from previous studies is proposed in this paper. The calibration method is verified by single element tests for unconfined uniaxial and triaxial compressive data. In addition, with the introduction of the dynamic increase factor (DIF), a finite element model is produced to demonstrate the validity and applicability of the proposed method through comparison with the experimental results of SFRC slabs subjected to projectile impact.