Abstract: Development of hydraulic machines with ultra-low head is a research challenge to scholars around the world, especially for the turbines used in tidal energy power station, which have strong influence on the ecology. According to the damage suffered by fish in turbines, the mechanical impact and pressure change etc., are the main factors. A kind of fish-friendly turbine with spiral blades based on variable pitch is proposed in this work, which has the advantages of simple structure, large conveyance capacity, powerful operation performance etc. Based on three-dimensional simulations with Computational Fluid Dynamics (CFD), this work aims at studying different parameters like blade number and pitch, as well as different operating conditions, to investigate the fluid characteristics with fishes.. The results show that fish-friendly turbine with two spiral blades (K_l=1.2:0.9) has a higher fish survival rate than traditional tabular turbine with several blades, and properly lengthening the runner channel is beneficial to reduce the pressure gradient (the length of blade area increased 3.9 times). In addition, the mechanical damage of the new runner with two spiral blades to fish is minimum. Numerical simulations are used to verify design model. When the turbine is running under the design condition, its efficiency is 83.43%, the maximum value of pressure gradient is −31kPa/s (lower than the threshold of suffering from pressure gradient damage, and its magnitude is −50kPa/s). Moreover, pressure damage probability to fish is only 0.02%, and this range is below the pressure damage threshold level for fish.