The problem facing of spool used in high pressure wellhead is illustrated. The methods and steps of conventional design and autofrettage design for spool are introduced. Based on the 4rd strength theory, the wall thickness of spool under 103.5MPa is designed using the two methods. Compared with the conventional design, the wall thickness of spool is declined 16.6%, the gross weight is 60% of previous, the stress distribution is more uniform and the utilization rate of raw materials is increased by autofrettage. Thus it provides a theoretical basis for design of spool used in high pressure wellhead.