Abstract: During the construction of offshore oil platform, a great quantity of braces and risers will be used. The girth welds involved in this kind of structure often require 100% ultrasonic inspection to determine whether there are defects in the weld. Due to the large allowance of roundness and misalignment, the ultrasonic echo forming in the root of weld is very complex. In this paper, the root ultrasonic echo caused by misalignment of butt girth weld of single side welded pipe is analyzed.