Abstract: In order to obtain cause of valve bolts cracking and prevent cracking accident occur once more, methords of ultrasonic detection, chemical composition analysing, mechanical property detection, microstructure analysing and SEM analysing were used to analyse the valve bolts of 600MW turbine. Results illustrate that it is effective to use ultrasonic technology to detect coarse grain of 20Cr1Mo1VTiB bolts. In the mean time, the coarse grain size of cracking valve bolts is responsible for reducing the plasticity and dynamic ductility of bolt heavily. The bolts shall be breaked in screw where heavy stress concentration occurs when frenquent impulse load is acted on it.