There are many stream processors on GPU. Compared with CPU, GPU has the ability of multi-threading concurrency. At present, GPU has been widely used in the fields of image processing, recognition, in-depth learning and so on, which need a lot of scientific computing. However, in the field of power system, the application of GPU is still in the initial stage of exploration, and there are few related studies. Firstly, this paper studies the architecture of GPU, analyses the multi-threading parallel ability and the characteristics of CUDA programming. Secondly, the mathematical model of six-pulse converter is established by using switching function. The harmonic part is calculated by GPU and distributed to many threads on GPU to solve in parallel.