In the field of aerospace defense, 2.5D C/SiC composites are crucial materials for high-temperature resistant components. The high hardness and wear resistance of these materials render the high-quality micro-hole machining exceptionally challenging. A picosecond laser drilling process for 2.5D C/SiC composites based on magnetic field/liquid-assisted machining (MLM) was proposed. By extracting the characteristics of micro-hole inlet/outlet diameter, taper, oxidation, and recast layer, a comparative study was conducted between MLM and three processes: picosecond laser machining (PM), magnetic field-assisted machining (MM), and liquid-assisted machining (LM). The results demonstrate that the picosecond laser machining based on MLM can effectively reduced the inlet diameter of the micro-holes while increasing the outlet diameter, achieving a micro-hole taper of 1.3° and 1.1° under two different sets of experimental parameters, the taper is reduced by 18.75% and 45% compared with PM process, reduced by 13.33% and 31.25% compared with MM process, reduced by 91.22% and 79.63% compared with LM process, respectively. Furthermore, microstructural analyses of the drilled micro-holes were performed using EDS spectroscopy, Raman spectroscopy, and XPS techniques. It was found that the MLM process more effectively reduced the graphitization defects on the hole walls. Among them, liquid-assisted machining avoids oxidation at the inlet of micro-holes, while clearly observing exposed fibers and matrix, effectively avoiding thermal damage and recast layer defects. In picosecond laser drilling, the primary mechanisms of MLM are attributed to the cooling effect of the liquid, its ability to isolate oxygen, and the longitudinal stretching of the plasma by the magnetic field, which reduces the plasma shielding effect.