Abstract: According to the standard JB/T 4730.3—2005, reference block of GS series for ultrasonic testing to girth welding joint of tube of diameter below 159 mm, was designed, and for tubes of diameter over 159 mm, only advanced requirements on it, however, was needed. Based on the analysis to these requirements, the design of reference block was concluded to three steps, and a design plan of reference block according to single or many dimensions of tubes and series of reference block in ultrasonic testing was put forward.