In this paper, we introduce a novel substrate noise estimation technique during early floorplanning for mixed signal system-on-chip (SOC), based on block preference directed graph (BPDG). Given a set of analog and digital blocks, BPDG is constructed based on their inherent noise characteristics to capture the preferred relative locations for substrate noise minimization. For each instance of floorplan in sequence pair or B*-tree, we efficiently count the number of violations against BPDG which correlates remarkably well with accurate but computation-intensive substrate noise modeling. Thus, our BPDG-based model can guide fast substrate noise-aware floorplanning and layout optimization for mixed signal SOC. Our experimental results show that the proposed approach is significantly faster than conventional full-blown substrate model-based floorplanning.