Aiming at the issue of the penetration threshold of 45# steel by irregular fragments, a ballistic gun test was conducted to investigate the penetration of a 6mm 45# steel target by irregular fragments. A 25mm caliber ballistic gun was utilized to fire irregular fragments into a 6mm-thick 45# steel target at different penetration angles to obtain the projectile-target shape and threshold velocity, and a threshold velocity model (the empirical formula and the THOR modified model) was established. The model was verified through the threshold test of 8g irregular fragments penetrating a 10mm-thick target. Combined with mathematical statistics, the THOR threshold velocity model was tested. The results indicate that approximately circular perforations are left on the front and back of the target plate after the penetration of irregular fragments. The average error between the calculation results of the threshold velocity model and the test results is less than 10%, which meets the requirements of engineering applications. However, the error of the modified THOR model is as high as 7.4%, and the calculation accuracy of the threshold velocity is higher. The results of this research provide a basis for the power optimization design and damage effect of the irregular fragment kill detonation warhead.