In view of the negative effect of harmonics on the safety, the economics, and the operation of electric power system, this work proposes an electric power big data based harmonic simulation system which is studied from the aspects of visual modeling, harmonic computation, and the combination of electric power big data and harmonic simulation system. Based on GEF and JavaFX, the grid network is visually built and the simulation results are graphically represented. By extending openDSS, traditional and new harmonic simulation models are implemented. Meanwhile, electric power big data is used as input, which can not only reduce the workload and error rate of manually inputting data, but also get better simulation results. Case study shows that the proposed system is able to support harmonic simulation, is more intuitive, is easy to be operated and extended, and is a new attempt to combing electric power big data and harmonic simulation system.