When the liquid rocket engine turbopump starts, the tank valve opens first before the pump is driven. Due to the tank and hydrostatic pressure, there is a passive flow phenomenon inside the pump called natural flow. The impeller is passively rotated by hydraulic impact, which impacts the stability of the turbopump during the initial startup period. In order to study the unsteady natural flow characteristics of the turbopump, the coupled CFD/6DOF solver is used for simulation and obtaining the variation of rotational speed, flow loss, and torque with time. The characteristics of the turbopump impeller"s radial and axial force are analyzed. The internal flow field and pressure pulsation features were investigated. The results show that the rotational speed caused by natural flow increases exponentially with time. Furthermore, the natural flow loss and torque gradually decrease with the increased rotational speed and has distinct pulsation characteristics and periodicity. The backflow phenomenon in the flow channel becomes more complex. The suction chamber and the inducer have a buffering effect on the impact of the incoming flow, making the pressure distribution more uniform. Due to the matching angle and the rotor-stator interference, the pressure pulsation in the guide vanes has distinct sub-harmonic interference during the natural flow process.