The invention relates to an arm-length-variable vertical shaft tide current water turbine generator set which belongs to the field of development of renewable energy sources. The generator set mainly comprises a blade, an arm seal, a telescopic arm, a piston, a fixed arm, a hydraulic cylinder for shortening, a hydraulic cylinder for extending, a support wheel disk, a compressing nut, a tide current speed and direction sensor, a main shaft, a main shaft bearing, a big gear, a support seat, a hydraulic oil station, a hydraulic pump for shortening, a hydraulic pump for extending, an electric brush, an integrated control system, a shaft neck seal, a small gear shaft bearing, a small gear shaft, a coupling and a generator. The telescopic arm is dynamically extended or shortened in the rotatingprocess of the blade according to the tide current direction and flow speed; and when the normal of the telescopic arm is parallel or perpendicular to the tide current direction, the telescopic arm is respectively changed to be the longest or shortest so that a vertical shaft tide current water turbine reaches highest energy capture efficiency. The self-start capability of the water turbine on low-speed tide current can be also improved by adjusting the telescopic arm. The arm-length-variable vertical shaft tide current water turbine generator set disclosed by the invention can be used for the comprehensive utilization and development of river water flow energy and ocean tide current energy.