Nuclear weapons are complex systems with special requirements on effectiveness, safety and reliability and, therefore, science-based engineering studies have been conducted, while enhancing system engineering and computer simulation. To further study mechanics and material science for environmental adaptability of the complex systems, theoretical and computational advances must be pursued. The most important issues are theoretical modeling and test designing. Unfortunately, appropriate theories and models for complex system have not been formed yet so far. Following the science development in the 21st century, applications of computer simulations will be speed up for complex systems investigations in various disciplines.