As a carrier of battlefield information, tactical data message is the core of data interacting in tactical command information systems.The simulation and generation of tactical data messages can be used to support training, joint debugging, software testing, troubleshooting and so on.Currently, army is widely used VMF (Variable Message Format) messages.Theoretically, based on corresponding VMF message standards, if we assemble message data to meet the requirements, VMF can be simulated.But today's simulation generation is too low and has a great dependence on on the original system database, which can not meet our needs in practical applications.This paper describes the architecture of command information system, and then analyzes the data transmission of command information system, finally, analyzes structure and characteristics of VMF and proposes a VMF encoding-decoding method.