At present, many DNA logic circuits have been successfully constructed. However, advanced complex DNA computing tasks with simple structure, fast response, and modularization still remain a huge challenge. In this paper, molecular switch circuits (MSCs) are constructed based on DNA strand displacement to improve above problems. At first, molecular switch is constructed, which is the basic element for building logic circuits. Next, the functions of AND, OR, Fan-in, and Fan-out are realized through switches cascading in series or parallel. Furthermore, switch canvas strategy is explored to obtain the shortest reaction path and highest output concentration for circuits. Finally, a novel strategy of integrating computing module using molecular switches is proposed. Here, we construct a half-adder, half-subtractor, and a 9-bit parity checking circuits to verify its feasibility. Compared with the classical monorail and dual-track logic circuits, the structure scale is reduced by more than two times, and the DNA strands are decreased by more than four times, which effectively reduces the circuit complexity and raises the reaction speed. The above results show that MSC strategy has a great potential to construct large-scale DNA circuits and provides a development model for the scalability and modularity of biological computing.