Abstract: A design software for rotary piercing mill plug is developed by analyzing the steps for designing the piercing mill plug and by adopting Lazarus software.The algorithm principle and the functions of the plug design software are introduced.This plug design software can quickly design a plug as well as draw the plug contour and the metal contour.It has characteristics such as convenience, universality and high precision and is able to do the calculation according to different plug types and working conditions.It can also store the plug design parameters as well as the production and quality data in the database, which can be used later as the basis of a new plug design.