L) The first stage, from the early 1960s to the late 1960s. The CAD system at this stage is characterized by using large general-purpose computers (mainly transistor circuits) and refreshing graphic display; The CAD system in this period is expensive and simple in performance. Only a few large enterprises in the world have developed or owned about 200 sets of CAD systems, which are mainly used in aviation and automobile manufacturing. The first stage, from the early 1960s to the late 1960s. The CAD system at this stage is characterized by using large general-purpose computers (mainly transistor circuits) and refreshing graphic display; The CAD system in this period is expensive and simple in performance. Only a few large enterprises in the world have developed or owned about 200 sets of CAD systems, which are mainly used in aviation and automobile manufacturing.

2) The second stage, from the late 1960s to the late 1970s. In the late 1960 s, DEC began to produce minicomputers with quite low prices. At the same time, cheaper memory tube displays have also been applied, and various forms of graphic input devices such as light pens and graphic input boards have also been put into use. The price of CAD system has been greatly reduced, which has prompted the formulation of CAD standard system table, which will become the guiding standard for the development and application of CAD software in China's construction industry. At present, some standards are correct.

CAD technology may develop and popularize rapidly. The CAD system developed in this period has been widely used in the design of integrated circuits and printed circuit boards in microelectronics industry. With the practical application of computer drawing technology, graphic database has been developed, and commercial CAD system has been applied and popularized in small and medium-sized enterprises.

3) The third stage, from the early 1980s to the early 1990s. In this period, cheap solid-state circuit random access memory appeared, and raster scanning display, mouse and electrostatic plotter produced realistic graphics. With the development of VLSI technology, microcomputers, super microcomputers and graphic workstations have been widely used. The rapid development of commercial graphics system makes CAD technology expand from developed countries to developing countries, from product design to engineering design, which indicates that CAD technology has entered a practical period. But at this level, we can only say that computer-aided drawing has been fully utilized. It is still a long-term and arduous task to realize the third goal put forward by MIT, that is, to truly realize computer-aided simulation and simulate all the work of people in the whole process of product or engineering design in the past, and there are many technical difficulties that need to be deeply studied and solved.

4) The fourth stage, that is, from the mid-1990s to the present. At present, computer technology is developing at an unprecedented speed, and the hardware revolution represented by Intel chip technology provides a more powerful means for the innovation of CAD technology. As the product of high-tech development, computer-aided design (CAD) is developing towards integration, cooperation and intelligence, which will surely bring about great changes in the new century.