Plume–surface interaction (PSI) during powered descent on Mars is a major risk to landing safety. PSI occurs when retro-thruster plumes impinge on regolith, eroding the surface, forming a crater, and ejecting particles that can degrade visibility and vehicle stability [1]. Japan is considering a Mars landing mission around 2035 that employs retro-thrusters, and quantitative design guidelines are required to mitigate plume–surface interaction (PSI). This study analyzes PSI using a coupled CFD–DEM solver, focusing on how thruster height affects crater formation and ejecta characteristics. The gas phase is computed with compressible CFD, while particle motion and inter-particle contacts are resolved by DEM. Two-way coupling is employed to capture interactions between the plume and particles. A parametric sweep of thruster height is conducted under Martian conditions. The simulations reproduce characteristic crater growth and particle ejection. At low height, a deep and narrow crater forms due to high stagnation pressure beneath the plume (Figure. 1a). In contrast, at high height, a shallow and wide crater develops through shear induced by near-surface radial flow (Figure. 1b). Consistently, ejecta is dominated by vertical dispersion at low height, whereas horizontal dispersion becomes dominant at high height. These height-dependent behaviors imply distinct operational risks. Vertically ejected particles at low height may strike the lower structure of a lander and cause damage, while a wide crater at high height may reduce touchdown stability. The present results provide physics-based insight to support landing-sequence decisions, including selection of a safer retro-thruster shutdown altitude for future Mars landing missions.