An efficient method for generating unitary matrix of quantum logic circuits is presented. First, the truth table is generated by using the operation rules of quantum gates in the quantum circuit, and then the unitary matrix of the quantum circuit is constructed according to the mapping relationship between the truth table and the unitary matrix. Traditional methods are to generate a matrix by using the topological transformation rules of quantum gates, and then multiply the matrix of quantum gates in the quantum circuit to construct a quantum circuit. When the quantum circuit is large, traditional methods involve the generation and product of many large matrices, which results in a huge time cost. The method proposed here achieves dimensionality reduction skillfully, which greatly improves the efficiency of the algorithm. Taking GT circuit and NCV circuit as examples, when the number of quantum lines is as high as 8 and the number of gates is 643, the speed of the proposed method is hundreds of thousands of times faster than that of the previous methods.