Recently, development on the new concept of advanced small modular reactor, so-called Autonomous Transportable On-demand reactor Module (ATOM) was started in Korea. Major design objectives of the ATOM are adopting Dry Air Cooling System (DACS) and developing passive safety systems. Among several passive safety systems, the Passive Residual Heat Removal System (PRHRS) is crucial since it prevents a core to be damaged by decay heat. Generally, it is known that a currently considered PRHRS has a limitation of cooling capability after all of the stored water is vaporized. In this study, the PRHRS coupled with the DACS (PRHRS+DACS) is suggested to overcome the limitation. The objectives of this study are to assess cooling capability of PRHRS+DACS and to investigate the effects of the design parameters using the MARS code. The simulation results show that the time for PRHRS to maintain its cooling capability is extended by applying the DACS. Furthermore, the effects of the environment temperature and heat transfer area ratio on the cooling capability of the DACS was confirmed.