Abstract: After analyzing the traditional process of dyeing and problems associated with it,the paper pointed out that nonlinear metering and dosing of assistants are required for the current dyeing technique.For this reason,an automatic control system of metering and dosing based on PLC(programmable logic controller) is designed in which the nonlinear metering and dosing of assistants is realized by combination use of the frequency changer and the PLC.A detailed discussion is made on the structure of the system,its working principle and process,and hardware and software design.The results of its application show that the dyeing quality is improved remarkably.