Domain specific design of reconfigurable architecture is a hard and time-consuming job. In this paper, a fast and effective domain-specific design method is proposed which mainly concludes a top-down subgraph enumeration algorithm and a heuristic identification process based on topological searching. A clustering and splitting algorithm is used to enumerate all the maximal valid subgraphs (MVSs) of the topological ordered source dataflow graph. These MVSs are then searched topologically to identify all the convex subgraphs for candidate functions. During the identification, heuristic rules are used for search space pruning. Candidate convex subgraphs are finally grouped into function sets for function unit design. Experiment results show that the subgraph enumeration speed is improved in most cases. With the combination of fast MVS enumeration and topological searching, a small number of candidate functions are identified, which shows that the proposed design method is practical and effective.