Under the background of the bottleneck of Von Neumann architecture, artificial synaptic devices inspired by biological nerve have gained extensive attention to accelerate peoples steps towards the era of artificial intelligence. In this study, the effect of hydroxyl group on the synaptic performance of p-type organic thin film transistor (OTFT) is investigated by plasma treatment on the surface of SiO2 dielectric layer. The synaptic behaviors of OTFTs with different plasma treatment time are simulated and compared, including Excitatory Post-Synaptic Current(EPSC), Paired Pulse Facilitation(PPF)and Long-term Potentiation(LTP). The generation of synaptic properties is due to the electrons being trapped by the hydroxyl groups introduced into the semiconductor layer/ dielectric layer interface by plasma treatment. With the increase of plasma treatment time, more hydroxyl groups on the surface of the dielectric layer make OTFTs exhibit better memory and retention capabilities, and higher linearity, which is more conducive to its application in brain-like learning and neural computing.