Abstract: Aiming at the gas outburst caused by roof first weighting at working face, the three-dimensional geometric model of goaf were established to investigate the relationship between roof subsidence and gas outburst. The law of gas outburst is presented by carrying out the dynamic subsidence of roof through the dynamic mesh function of FLUENT software, which shows that the air pressure over the goaf increases first when the roof subsidence begins, the air which is rich in gas with high concentration flow toward working face due to high pressure airflow, which leads to the gas concentration of working face and ventilation roadway increase sharply. The gas concentration of working face increases and reaches maximum after the roof subsidence, which is known as "delay effect". The gas outburst after weighting can be divided into three stages, which is known as the gas concentration increases sharply, decay sharply and decay slowly.