In recent years, in order to handle various sources of noise (including substrate and power supply noises) and process variation in high-end mixed-signal circuit design, analog circuits are often required to be placed symmetrically to the common axis, and high noise digital circuits need to be placed far away from noise interference to the analog blocks. In this paper, we obtain the mixed-signal SoC f oorplan with the twophase approach. In the f rst phase, we place the symmetry groups and non-symmetry blocks by sequence pair representation with improved implementation. In the second phase, we obtain a f oorplan with minimized digital blocks noise interference to analog blocks by the effective decap f lls with substrate noise model. We have compared our experimental results with the recent works in symmetry constraints and mixed-signal SOC f oorplan with minimized substrate noise. The results demonstrate the effectiveness of our approach.