Abstract: With SEM and Energy Spectrum to study the unqualified reason by ultrasonic testing of X70 pipeline steel plate. The experiment confirms that the unqualified district was formed by large amount of and size more than 100 μm gas cavity and shrinkage porosity which were pressed and combined in rolling process;the gas cavity was caused by argon gas for protection which was at collector nozzle during the continuous casting. Shrinkage porosity was caused by higher drawing speed for continuous casting . The experiments conclusion was that with closing the argon for protection at collector nozzle and lowering down the speed can make qualified rate to reach 100% during ultrasonic testing of X70 pipeline steel plate.