From the point of view of the optimization in fuzzy control, the idea of variable domain and the concept of flex factor are discussed. At the same time the preponderance of variable domain and the relationship between the quantization factor, the proportional factor and the flex factor are explained. In addition, the method of selecting the two flex factors is analyzed in detail. Another new intelligence control means - fuzzy logic adjusting PID online based on variable domain on self-adaptation is realized by putting the idea of variable domain into fuzzy PID. The research is based on some complicated nonlinear system, and the result of simulation is predominant.