The van der Waals integration of atomically thin two-dimensional (2D) materials can enable a new generation of tunable optoelectronic devices, in which the photocarrier generation, separation and extraction can be modulated by an external gate potential. However, studies to date only show a limited modulation due to non-ideal band alignment and the lack independent modulation of distinct 2D layers in the van der Waals heterojunction. Here report the construction of a WSe2/GeSe heterojunction photodiode with type-II band alignment, in which the value of the conduction band and valence band edge in WSe2 is approximately equal to that of GeSe. What's more, the Fermi level of GeSe remains relative stationary and that of WSe2 can be modulated across the entire band gap by an external gate-voltage, thus rendering widely tunable open-circuit voltages from positive (+ 0.7 V) to negative (− 0.1 V). We further show the photoresponsivity can be modulated by the gate-voltage with a factor of 105, which is similar to the current switching ratio in traditional field-effect transistors, opening up exciting potential for photonic logic circuits.