This paper presents a modular multi-magnetron unit design based on the mutual coupling phase-locking scheme, configuring it as an axially output module to address the issue of azimuthal non-uniformity in radial output magnetrons in large-scale array applications and significantly improve system robustness. The magnetrons are connected via phase-locking bridges, and the traditional designs based on single magnetron modules lack flexibility. Additionally, due to the non-uniform azimuthal electric field distribution of radial output magnetrons, adding coupling structures to the phase-locking bridge at different positions causes changes in the characteristic impedance of the magnetrons and alters the proportion of energy in the phase-locking bridge relative to the total system energy, thereby affecting output characteristics and phase-locking efficiency. Initially, various topological structures of magnetron phase-locking units were designed and simulated, showing that the opening position and number of the bridge significantly impact phase-locking performance due to the azimuthal field uniformity. Subsequently, an end-to-end series module, suitable for modular design, was selected for further study, and a four-in-one power combiner was designed based on this structure, making the unit output axially. After power synthesis, the output power of the four-tube phase-locking modular unit reached 988 kW, approximately four times that of a single free-running magnetron, with a phase-locking efficiency of 97%.