In order to study the start-up characteristics of liquid oxygen methane full flow staged combustion cycle engine(FFSC), a comprehensive simulation model of the engine system is constructed based on the general simulation software AMEsim model library. The start-up process is simulated to investigate the influencing factors. The simulation results show that the full flow staged combustion cycle engine exhibits a significant start-up overshoot phenomenon, with the pressure overshoot in the main combustion chamber significantly higher than that in the two preburners. Delaying the ignition time of oxygen-rich preburner can effectively reduce the pressure overshoot in the main combustion chamber, but it will increase the pressure adjustment time and decrease the starting speed. The maximum power of the forced start-up device should not be less than 40% of the steady-state power of the turbopump, as excessive power or prolonged operation of the device will increase the start-up overshoot.