Abstract: Expanding application of computer-aided design (CAD) technique to the particle-in-cell (PIC) simulation will greatly make a large space towards the practicability of PIC method. Using object-oriented and modularized design methods, a three-dimensional CAD system for PIC simulation is realized based on the analysis of the requirement of modeling complex vacuum electronic microwave sources, millimeter-wave sources, and terahertz sources. The CAD system can visually model various complex devices and generate the parameter description file for the kernel calculation. Using this system, a magnetically insulated transmission line oscillator (MILO) and an extended interaction oscillator (EIO) are modeled and simulated in cylindrical and Cartesian coordinates respectively. The results show the practicability of the three-dimensional CAD system for PIC simulation.