In this paper, a novel compact high-voltage (HV) test system based on a magnetically controlled resonant transformer (MCRT) is proposed. This system is used to perform a power-frequency HV on-site voltage withstanding test. First, the basic principle of the MCRT is introduced. Next, a Fourier analysis based on the equivalent magnetic and electrical circuits of the MCRT is described. This analysis provides a design reference for the basic MCRT parameters. In addition, a finite-element model was constructed to verify the MCRT design. Finally, an MCRT prototype was designed to verify the effectiveness of the test system. Compared with the traditional HV test systems, the proposed test system in this paper has the advantages of compact structure, stable secondary voltage, and wide reactance adjustment range.