PLC control system for sulphonation process shows the superiority of PLC over ot her kind of computer process control. It does not need the operator to press the button and turn the valve to control the temperature and regulate the velocity of sulphuric acid. In this paper, the connection method of PLC analogue expansio n I/O unit is presented, and the collection, computing, processing, control of a nalogue data and relevant programs in the system are also presented. The method of fuzzy logic switch control presented in the paper is economical, efficacious and reliable in practice.