In order to analyze the transient characteristics of tubular pump during non-regulative operating condition, a shaft-extension tubular pump with rapid-drop gate and slap doors attached was established, and a 3D numerical simulation for start-up process was investigated numerically. The rational speed was deduced by utilizing the moment balance equation. The movement of the gate with additional slap doors was specified based on the laying mesh and dynamic mesh technology. The simulation results showed good agreements with the test results. The result showed that the starting speed and the change of flow rate had obvious influence on the pressure gradient in the impeller section. The pump head was increased firstly and then decreased to the rated head during the start-up process. The maximum start head was 6.38m at the time of 2.25s, and the maximum pressure difference between front and rear of the slap door was 2.61m. As a result, the outlet gate with additional slap doors could effectively reduce the maximum start head and improve the safety factor of the pump unit. Meanwhile, the additional slap doors on the gate played a good role in the shunt during the start-up process, which can avoid the system instability caused by excessive water pressure in the outlet passage when the impeller speed rose fast and the gate was not fully opened.