Abstract: During the long test to a routine inspection of borescope detection of engine, a suspected penetrating crack on the free turbine blade was dected by conducting magnetic flux and fluorescence fault confirmation. There was a crack with a length of 5~7 mm at the trailing edge of one blade. After returning to the plant for decomposition and fluorescence examination, cracks were found in many blades. Though the macroscopic examination, fracture analysis and metallographic dissection of the cracks, the cracking properties of free turbine blades are determined, and the cracking causes of free turbine blades are analyzed and verified. The results show that fracture of free turbine blade belongs to fatigue cracking. From the calculation and load spectrum review of free turbine blades, it shows that the rotor couplings resonance with the blades within the working speed range due to the change of load spectrum in the long test, which results in the blade height bending fatigue crack.