Abstract: It is assumed that the defect is located in the center of the weld in parallel scan by TOFD, so the measurement of orientation parametters such as depth and height has some errors as for the axis deviation defects in weld. Through accurate mathematical deduction, the article establishes detailed mathematical model so as to analyze the changes regulation of measurement error of depth and height by various factors (such as true depth, true height, axis deviation, probe center separation),and experimental verification by TOFD block is carried out; meanwhile, the article proposes the methods of reducing the measurement error, and they have great guiding significance for process selection of TOFD nondestructive testing.