Abstract: While measuring the thickness for a pressure vessel steel plate abnormal display often occurred with the thickness at the plate central being about half of the actual value. A series of tests, including ultrasonic testing, surface inspection, mechanical properties test, acid etch test and radiographic detection method tests, were conducted to analyze the cause of the abnormal display. Results show that, the abnormal phenomenon appearing in ultrasonic thickness test is caused by the plate low times microstructure and is the result of reflecting tissue. And the unqualified lateral bending is due to the segregation of steel plate center. The above-mentioned results may provide reference for the steel mills to strengthen smelting process and smelting technology to improve and control the steel quality.