Abstract: In recent years, the recycling and utilization of low-head river water energy for remote mountainous areas—particularly for water supply, power generation, irrigation, and other livelihood projects—have garnered significant attention. The ultra-low head axial flow hydraulic turbine has become widely adopted due to its ability to operate efficiently under high-flow, low-head conditions. However, the dynamic nature of driving loads, inflow instability, and the presence of multiphase media often lead to speed instability in these turbines. In severe cases, this instability can disrupt the normal operation of the driven load. During model tests of ultra-low head axial flow hydraulic turbines, rotational speed fluctuations under design conditions have been observed to reach up to 20%, resulting in unstable power output. To investigate the underlying causes of this issue, this study focuses on a specific type of ultra-low head axial flow hydraulic turbine and employs the ANSYS Fluent2020R2 dynamic grid SDOF (six-degree-of-freedom) solver for numerical simulations. The research systematically examines the effects of load torque variations, flow rate changes, and gas content on the turbine's rotational speed characteristics. The results show that during startup, the torque coefficient rapidly peaks, undergoes brief fluctuations, and then stabilizes, while the rotational speed sharply increases, experiences minor oscillations, and gradually stabilizes. At steady state, the simulated rotational speed decreases nonlinearly with increasing torque and remains consistently lower than the theoretical speed, with the discrepancy widening at lower torques. This is attributed to higher internal flow velocities at lower torques, increasing frictional losses and reducing recovered power, thereby lowering the rotational speed under constant torque conditions.The turbine's rotational speed closely follows flow rate variations, exhibiting a positive correlation regardless of whether the flow increases linearly or fluctuates sinusoidally. Higher flow rates increase both the turbine head and head loss, resulting in relatively stable efficiency with minor fluctuations and short cycles, indicating uniform efficiency distribution during flow variations. Gas volume fraction significantly impacts performance: increasing it from 0% to 30% reduces rotational speed and power coefficient by 16.2% and 16.4%, respectively, while a rise from 5% to 30% decreases efficiency by approximately 4%. This highlights the detrimental effect of gas content on energy conversion efficiency and operational stability. The study also explores the internal hydraulic losses and efficiency variations under different operating conditions. For instance, during torque fluctuations, the turbine's average efficiency remains around 81%, with water head losses primarily driven by frictional effects. Similarly, flow rate fluctuations lead to efficiency variations of approximately ±10%, which is half the amplitude of the flow rate fluctuations. These insights underscore the importance of maintaining stable inflow conditions and minimizing gas content to ensure optimal turbine performance. The findings of this research provide valuable guidance for the design and operation of ultra-low head axial flow hydraulic turbines. By understanding the relationship between torque, flow rate, gas content, and rotational speed, engineers can develop more effective control strategies to mitigate speed fluctuations and enhance power output stability. This study serves as a foundational reference for future research and practical applications in the field of low-head hydropower systems, particularly in remote and resource-constrained environments.