对单面减薄和全截面试样进行拉伸试验，通过金相、拉断试样加工面和断口的检测分析，讨论了单面减薄对 Q420B厚板断后伸长率的影响。结果表明： Q420B厚板单面减薄试样的断后伸长率较低，拉断试样中心存在显微裂纹；断口为韧 -脆混合型，但与全截面试样相比，单面减薄拉伸试样脆性区占断口截面比例大，断口呈 “V”字型。提出了进行厚板拉伸试验时，在满足标准要求的情况下，优化加工和试验方法，比如调整宽度或适当减薄或加强试验机能力等。

Tensile tests were carried out on single-side thinning and full-section specimens.By analyzing the metallography，the machining surface with tensile failure andfracture，the effect of single-side thinning on the elongation after breaking of Q420B thick plate was discussed. The results showed that the elongation after fracture of the single-side thinning sample of Q420B thick plate was lower. There were microcracks in the center of tested sample. The fracture was a mixture of toughness and brittleness，but compared with the full-section sample，the brittle zone of the single-side thinning tensil especimen accounted for a large proportion of the fracture section and the fracture was V-shaped. It was proposed，when the thick plate tensile test was carried out，the processing and test methods were optimized under the condition of meeting the requirements of the standard，such as adjusting the width of the sample or properly thinning or strengthening the capacity of the testing machine.