Torque generation and flow distribution of a lift-based vertical-axis turbine with an upstream deflecting plate are investigated in water tunnel experiments. The deployment of a deflector in front of a lift-based turbine is a promising approach to increase local flow velocity and enhance energy conversion efficiency without consideration for complicated control. For the turbine with the deflector, the phase during which the blade passes near the front end of the turbine has a major contribution to torque increase from the case without the deflector. Meanwhile, the deflector can have a negative effect in torque generation at the phase when the blade moves upstream against free stream if the turbine is placed close to the deflector in a crosswise direction. The change of nearby flow distribution by the deflector is also examined to find its correlation with torque generation. When the blade rotates through the near-wake region of the deflector, the blade can collides with the vortical structure shed from the deflector. This interaction causes significant torque fluctuation.