Neuromorphic devices, which break the traditional von Neumann architecture, have attracted much attention for their ability to mimic the perception, learning, and memory functions of the human brain. Two-dimensional (2D) materials are excellent candidates for building neuromorphic devices due to their atomic-level thickness and excellent optoelectronic properties. In this work, we designed a nonvolatile floating-gate synaptic device based on an HfS2/HfOx/SnS2 van der Waals heterostructure. This device exhibits a large memory window of 60 V, based on which synaptic properties, such as an excitatory postsynaptic current, and short-term and long-term plasticity, were simulated. In addition, the long-term potentiation/depression (LTP/D) characteristics of the device can be optically modulated. The device has a low nonlinearity of 0.22 for LTP, and the ratio of the number of effective conductance states was 93.3% under 532 nm illumination; this is an improvement on the levels reported using 2D-material floating-gate devices in recent years. This work offers the possibility of future applications of optoelectronic synaptic devices.