In this Letter, we report an optically stimulated synaptic transistor based on MoS2/quantum dots mixed-dimensional (MD) heterostructure, where the channel conductance shows a non-linear response to the optical stimuli. Paired-pulse facilitation is realized with the index above 200%, and the optical synaptic plasticity can be modulated by adjusting the amplitude, duration time, frequency, and power of light spikes. In addition, the long-term plasticity shows a gate-tunability, which can be attributed to the unique photoelectric coupling in MoS2/quantum dots MD heterostructure. This work opens up a new way to explore optically stimulated synaptic devices based on designed device structure and provides a feasible method to achieve plasticity modulation by gate voltage, which plays an important role in developing neuromorphic devices with complicated functions.