Lahar is one of most serious volcanic disaster, threats people’s lives and property security of volcanic region. The numerical simulation result can predict the characteristics of lahar, provide reference materials for making disaster prevention plan, and make the measure of lahar disaster relief transition from passive to active. Selected the area near the Tianchi as the research region, and divided the research region into forming region, circulation region and accumulation region. In order to establish the calculation model, divided the circulation region and accumulation region into many meshes, which are small enough to meet the calculation accuracy. The calculation time step length is also identified very small as 0.001 min. Calculated the average and peak velocity of lahar, triggered by the assuming medium and small eruption of Tianchi volcano, through the Delft 3D software. The longitudinal variation of lahar frontal average and peak velocity is acquired by the numerical simulation result. The time to area inundation by lahar, triggered by medium eruption, is obtained by calculation. The lateral variation of lahar depth at each cross-section is simulated. Although there is some different between the assumption of this research and the actual condition, flow velocity and accumulation situation of lahar are calculated in quantity and extension by numerical simulation, these result has reference value for preventing and reducing lahar disaster.