The invention relates to a design method for a controllable transient-flow leakage detecting system. The design method comprises the following steps: calculating an on-way resistance coefficient of ato-be-detected pipeline; calculating an on-way resistance coefficient of a connecting pipeline; collecting data of a to-be-detected pipeline; determining strength of controllable transient flow duringdetecting; calculating flow speed changes of the to-be-detected pipeline; calculating pressure of a transient-flow excitation device; calculating initial compression amount; calculating the volume ofa pressurizing cavity; calculating the diameter of a pressurizing piston; and calculating length of the pressurizing cavity. The system designed by the design method disclosed by the invention can beapplied to leakage detection of water conveying pipelines of hydraulic engineering, municipal water supply and the like, provides theoretical support for the controllable transient-flow exciting device, and also provides a technical base for controllable transient-flow leakage detecting.