ActarSim is the simulation code proposed for the ACTAR detector simulation. It is based on GEANT4 and ROOT. ActarSim uses the GEANT4 tracking capabilities to follow the energy loss in the detector gas of the ions created in the reactions of interest. The two-body kinematics and simple beam-interaction model are included. Once the energy loss is calculated, additional macros are provided for the visualization and digitizacion in the pad plane. The code is flexible to manage the different possible configurations under study.