To study the influence of particle size on critical detonation performance of hexanitrohexaazaisowurtzitane (CL-20)-based explosives ink by micro-flow direct writing technology, three different particle sizes of CL-20 explosives were prepared by mechanical ball grinding method and solvent-nonsolvent method, respectively. The CL-20-based explosive ink by micro-flow direct writing technology was prepared by a two-component adhesive dispersion system consisting of waterborne polyurethane (WPU) and ethyl cellulose (EC). The particle size distribution and morphology of the prepared CL-20 explosives and the corresponding ink samples were characterized by laser particle size analyzer and scanning electron microscope (SEM), respectively. The crystal type of the CL-20 in ink samples were measured by X-ray diffractometer (XRD). The critical detonation thickness of CL-20 explosive ink samples with different particle sizes were tested by wedge shaped charge test. The results show that the two CL-20 explosives prepared by mechanical ball grinding method have a nearly spherical shape and smooth surface, whose median size are 140 nm and 1.5 μm, respectively. The corresponding ink samples have a honeycomb shape, with good dispersing effect and uniform compact cross-section. The CL-20 explosive prepared by solvent-nonsolvent method has a fusiform shape with a median size of 15 μm. The corresponding ink sample has poor dispersing effect, and the explosive particles and adhesive system are difficult to form a composite structure. The XRD test results indicate that three crystal types of the CL-20 in ink samples are all ε form. The critical detonation thickness of CL-20 explosive ink samples with a charge width of 1 mm is as small as 69 μm, and decreases with the decrease of CL-20 particle size, indicating that reducing the particle size of the CL-20 can significantly enhance its critical detonation performance.