A cap-plate logging while drilling tool experienced internal leakage in downhole operation due to the penetration crack between the inner channel and the T-groove bottom. The original location of the crack was determined through analysing the morphology of the penetrating crack， and then the cause for the penetrating crack was determined through the macro- and micro-analysis of the fracture. Finally， the structure of the tool was optimized with the finite element analysis to improve the service life. The analysis results show that the presence of large stress concentration and the cutting marks at the bottom edge of the T-shaped groove led to the fatigue penetration crack， which causes the intrusion of drilling mud， and the crack originated from both sides of the through bore at the bottom of the groove. The stress distribution can be improved by optimizing the groove bottom fillet and other measures.