The paper introduces Petri net knowledge into expressing electromechanical equipment virtual assembly or disassembly process, and on the basis of it builds the virtual assembly or disassembly models. In view of problems as genetic algorithm falling into the local maximum too early and slow searching speed, chaos searching will be used to process initial population of genetic algorithm, so as to improve the efficiency of genetic algorithm to search optimal virtual assembly sequence and optimize the evaluation scheme. Finally, taking a multifunction machine tool in existing virtual system as an example to verify the method is efficient.