The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel(C110 steel) were investigated. The studied casing specimens are as follows: original casing, plastic deformation(PD) casing and PD casing after being immersed in NACE A solution saturated with H2S(PD+H2S). Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness(JId) was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of the casing decreases after plastic deformation. Compared with that of the original casing and PD casing, the dynamic fracture toughness decreases further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows: the plastic deformation results in the increase of defect density of materials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreases greatly. The effects of plastic deformation and H2 S on fracture toughness of high strength casing steel (C110 steel) were investigated. The studied casing specimens were as follows: original casing, plastic deformation (PD) casing and PD casing after being immersed in NACE A solution saturated with H2S (PD + H2S). Instrumented impact method was employed to evaluate the impact behaviors of the specimens, meanwhile, dynamic fracture toughness (JId) was calculated by using Rice model and Schindler model. The experimental results show that dynamic fracture toughness of compared with that of the original casing and PD casing, the dynamic fracture toughness downshown further when the PD casing immersed in H2 S, moreover, there are ridge-shaped feature and many secondary cracks present on the fracture surface of the specimens. Impact fracture mechanism of the casing is proposed as follows: the plastic deformation results in the increase of defect density of ma terials where the atomic hydrogen can accumulate in reversible or irreversible traps and even recombine to form molecular hydrogen, subsequently, the casing material toughness decreased greatly.