A software security module is designed,which is an external hardware module,and can provide such functions as copyright authentication,kernel data encryption,stable operation monitoring and so on.The external module uses USB to connect with host PC,and a free driver portable data channel is achieved by taking the microcontroller of security module as a virtual USB mass storage device.A communication protocol is designed for use between the security module and PC based on packets layered communication protocol stack,and thus to implement safe and reliable data communication.The usage of the security module is illustrated by taking the software for cable ampacity analysis as an application example.