The Institute of Modern Physics, Chinese Academy of Sciences performs research and development on the target station of an accelerator-driven system (ADS) under the China ADS project. A newly developed Monte Carlo program for the design of the target station named GMT1.0 is presented. The program is designed for a massively parallelized simulation of the initiative granular-flow target concept. Based on the combination of the Intranuclear Cascade of Leige (INCL) model and the ABLA evaporation/fission model, GMT1.0 integrates a particle transport code and a nuclear reaction code to simulate a spallation target. For validation, a series of calculations of neutronics characteristics and heat-deposit distributions of solid targets were performed, and a high degree of accuracy was shown for GMT1.0. Using GMT1.0, a systematic study of the neutron economy of the target was performed and the neutronics characteristics of the most optimal parameters were illustrated well.