In order to study the rule of the penetration threshold of irregular fragments into 45# steel, a 25mm caliber ballistic gun is used to launch irregular fragments into a 6mm 45# steel target at different angles, and the failure morphology of the target plate under different penetration conditions is obtained. The target-hitting velocity and residual velocity of the fragments are measured by the measuring speed of the paper targets, and the threshold velocity is obtained by analysis. The models (the empirical formula and the THOR modified model) of the threshold velocity of irregular fragments penetrating into a 6mm 45# steel target is established. On this basis, the models are verified by the threshold test of 8g irregular fragments penetrating into a 10mm steel target , and the THOR threshold velocity model is tested by mathematical statistics. The results show that the front and back of the target plates are left with approximately circular perforations after the irregular fragments penetrate. The threshold velocity of irregular fragments penetrating into 45# steel has a large fluctuation range. The average error between the calculated results of the empirical formula of threshold velocity and the calculated results of the THOR modified model and the experimental results is less than 10%,but the maximum single error of the THOR modified model is smaller, and the THOR modified model is suitable for irregular fragments to penetrate into 45# steel target with higher accuracy. The research results are convincing and reliable, which provides a basis for the power optimization design and damage effect of irregular fragment blasting warhead.