The flow condition of the downstream stilling basin of the low-head sluice is complex, and the outflow conditions of the sluice (free outflow, submerged outflow) have a great influence on the flow regime and water jump length of the downstream of the sluice. Based on the hydraulic model test, the calculation methods of the downstream stilling basin length were proposed under the operating conditions of sluice gate fully opened and maximum discharge：First, the length of the stilling basin can be calculated by using the existing design specification method under free flow conditions. Second, the length of hydraulic jump is shorter or no hydraulic jump in the downstream of the sluice under submerged outflow conditions, and then the length of the stilling basin can be calculated by using the upstream normal water level and the maximum opening discharge on gate controls of the sluice. Our results can be used to calculate and select reasonably downstream stilling basin length of low head sluices, and on the premise of ensuring the safe operation of the project, saving greatly the project investment.