Abstract: Abstract: Tubular turbine is an important part of the rural small hydropower electrical net. Owing to the low water head and large flow quantity of tubular turbine, the efficiency of power station units as well as the output value is low. The actual operation efficiency of similar units varying from 3 to 5 blades from home and abroad is between 84% and 88%, which is far from meeting the corresponding design requirements. In addition, the current studies of optimal working condition of tubular turbines are limited to a certain operating point or curve. From the perspective of runner hydraulic power loss, the typical tubular turbines on the Yellow River are chosen as study objects in this paper. In the meantime, we have done theoretical analysis on the factors influencing the efficiency of tubular turbines, such as aerofoil loss of resistance, ends loss of resistance and impact loss of resistance. According to the actual shapes of blade and vane, with the water head of 8 m and the chosen blade angle of 10°, by changing the turbine inlet flow angle from 39.2 to 58.8°, the velocity triangles of the inlet and outlet positions are calculated. Based on this theoretical analysis, it turns out that the most stable operating range for tubular turbines is between 56.6 and 58.8°, and the non-optimal range is between 39.2 and 51.1°. The optimal range is also the area showing the highest efficiency. In order to verify the correctness of this analysis, CFX technology has been applied to carry out numerical simulation of the working condition of tubular turbines. During the process, we have taken the isometric model calculation and analysed the flow field. It is found that vortex and secondary flow appear in low efficiency and non-optimal range, which is very unstable and will generate great loss. The flow regime in the high efficiency range is relatively stable. This result on optimal operation range fits the outcome of theoretical analysis. Based on this, real machine experiments have been carried out, using the index method of relative efficiency test. By measuring the runner pressure difference, index flow has been calculated to substitute actual flow. Finally the relative efficiency and the output value of the turbine are calculated to pinpoint optimal efficiency. The result of real machine experiments accords with theoretical analysis. The error between simulation and real machine experiments is smaller than 2%. At last, by calculating the whole working condition of the targeted station units using this method, we have proposed the optimal operating area within overall working condition range and improved the actual efficiency by 6%. The pressure fluctuations have been significantly reduced. The effect is distinct after long-time operation. By expanding from point and line to area, the research method provides evidence for the design proposals of optimal working conditions of real machine experiment. This has rendered effective solutions for the safe and highly efficient operation of tubular turbines.