Fieldbus has become one of the most active fields in industrial data bus area. However, fieldbus standards are not uniform at present, which brings many difficulties to different industrial control equipments from different manufacturers to connect their devices to others. CAN bus and Modbus bus are two kinds of more popular buses in industrial control systems. This paper analyses these two bus protocol, proposes a protocol conversion interface designing methods, and gives a detailed hardware and software designing process, which solves these two different bus devices connected issues.