A compact repetitive pulse power source is developed as an experimental platform for high power relativistic magnetron with low magnetic field. To obtain better output pulse waveform with a compact structure, the pulsed power source designed based on PFN-Marx technology has a coaxial structure. A circular pulse forming net (PFN) is devised out with the impedance of 4 Ω, working voltage of 50 kV, and electrical length of 53 ns, consisting of 13 ceramic capacitors with the capacitance of 1nF. Two PFN devices in series by a gas switch and an insulation plate form a circular high-voltage pulse generation module. Multiple pulse generation modules are coaxial and stacked in a metal cylinder. Inductive isolation is used between the modules. After all switches are turned on, all modules are discharged in series to generate a fast rising-time high-power square wave pulse. Moreover, repetitive operation is achieved through synchronous control of the trigger switch and charging power supply. In experiments the 22-stage PFN-Marx pulsed power source developed was charged to 51 kV, and a high-voltage square wave pulse of 516 kV was obtained on a load of 84 Ω, with pulse width (FWHM) of 104 ns, flat top of 63 ns and rising-time of 11 ns. This power source can operate stably at a repetition rate of 20 Hz for 15 s.