Harvesting photoexcited hot electrons in metals promises a number of benefits in optical sensing. In practice, hot-electron optical sensors with tunable performance in electrical sensitivity are still absent. Herein, we propose a design to realize tunable hot-electron optical sensing. The proposed device consists of a one-dimensional grating deposited on a planar Au-MoS2-Au junction that is used for efficient hot-electron harvesting. Photoelectric simulations show that when grating-assisted plasmonic resonance is excited, bias voltage between two Au layers can be used to manipulate the magnitude and polarity of responsivity at the working wavelength. Therefore, the change in responsivity that originates from the change in refractive index of analyte in which the device is immersed can also be tuned by applied voltage. It is found that when bias voltage is 1 V, the electrical sensitivity doubled compared with that when applied voltage is absent. We believe the bias voltage-tuned strategy that is applied to planar hot-electron harvesting junctions facilitates the development of optical sensing.