Abstract: The inner wall of pressure vessels are often subjected to the impact of high temperature, high pressure and corrosive media, which leads to the crack, pores, corrosion and other defects on the inner walls. Therefore, it is necessary to carry out the nondestructive testing and evaluation on the defects at the inner walls of pressure vessels. In this paper, the magnetic needle probe method is used to detect the defects at the back of structural steel samples with different defect sizes under the effect of excitation magnetic field. The hysteresis loops at the testing lines are analyzed, and the remanence B_r and coercivity H_c are extracted preliminarily to reflect the size of defects. It can be found that the magnetic signals near the defects show the obvious valley characteristic, and the width of valley increases with the increase in size of defects. In order to further evaluate the defects at the back of structural steel accurately, the area ratio S_r is proposed as a new characteristic value, and a linear relationship between area ratio S_r and size of defects at the back of structural steel is established. The research results can provide a guidance for the evaluation on the internal defects of pressure vessels by the magnetic needle probe method.