Abstract: In order to coordinate with Soft PLC developing system running on PC to accomplish corresponding control function, compiling system of Soft PLC was developed. Firstly, the paper briefly described the constitution of soft PLC compiling system as well as ergodic algorithm of soft PLC ladder diagram and instruction list. Secondly, it introduced interconversion implementation process of soft PLC ladder diagram and instruction list. Via examples, it expounded design process of instruction list grammar of soft PLC, PLC grammar predictive analysis sheet, predictive resolver model as well as its control program algorithm. In addition, the process of soft PLC program syntax analysis was elaborated and interpretation process of several common instructions was illustrated. The emulation results show that the compiling system developed can fulfill interconversion between ladder diagram and instruction list, and complete grammar design as well as syntax analysis of soft PLC program and give correct interpretation of program instructions, which can make PC carry out corresponding control.