Ba impurity in potassium dihydrogen phosphate (KDP) is studied with the first-principle simulation method. The relaxed configurations and density of the states of KDP crystal with Ba impurity are calculated. We find that Ba can generate a K vacancy and an interstitial O-H unit for charge compensation. The band gap of KDP crystal narrowed down to about 3.9 eV, which is consistent with the experimental data from previously reported studies and indicates that Ba may be a source of low-damage threshold.