Abstract: The dynamic fracture behavior of notched semi-circular bending(NSCB)subjected to hammer impact loading was studied by using caustics experimental method.The results show that the shear stress at the crack tip increased with the offset distance increased,and correspondingly,the crack changed from mode I to mixed mode I-II,the time from impact force loaded to crack initiation also increased,moreover,the crack deflecting angle became more fluctuating with crack propagation;the crack velocity increased rapidly under dynamic load until it reached a certain value with continual fluctuation,with the ratio of crack vertical displacement to the total vertical length λ＞0.8,the crack velocity sharply decreased because of the effect of the local compressive stress around the hammer;as the offset distance increased,the initial fracture toughness K_IC decreased,whereas K_IIC increased;while the dynamic propagation toughness increased gradually with crack propagation.