Superconducting magnet is the core of a superconducting magnetic energy storage (SMES) system, and its optimal design can improve the economical efficiency and the operating performance of SMES. This paper proposes a design method of high temperature superconducting (HTS) magnet considering the magnet dynamic characteristics. The finite element method and genetic algorithm are used in the design. Optimal variables are the inner radius of the magnet, the turns of a single pancake and the numbers of the double pancake. The optimization procedure is divided into two parts. The first part is optimizing the length of the HTS tapes, and the second part is optimizing the AC loss based on the results of the first part. Last, a 150 kJ HTS magnet is optimizd with this method, and the final magnet scheme has the advantages of less usage of the HTS tapes and low AC loss.