Abstract: Features and usage of S7-200 integrated closed-loop control instructions were described,application of S7-200 PLC and Touch Panel 37 in improvement of the automatic control system of YG1906 vacuum pre-conditioner was discussed,and a working plan for software and hardware design of the control system was established.Satisfactory results obtained by improving the system were that the vacuuming time per batch was decreased by 5-6 minutes,steam consumption and downtime were reduced,and working efficiency was improved.