The invention provides an automatic generation method for a Modbus slave-station simulator system. The slave-station simulator system is customized from the aspects including external behavior description, Modbus slave-station information description and external characteristic description. The slave-station simulator system possess excellent equipment similarity, which can substitute equipment for consistency and interoperability tests before and after production. The method can not only customize operation program of the slave-station simulator system but also generate source codes, wherein the source codes can be directly applied to development of a protocol device. The automatic generation method of the invention can reduce the complexity of protocol engineering, and has great significance in automation of the protocol engineering.