Objective To investigate the regulation of synaptic plasticity by cannabinoid receptor 1 (CB1R) and its effects on autism spectrum disorder (ASD)-like behavior. Methods CB1R-knockout (KO) mice and valproic acid (VPA)-induced ASD model mice (VPA mice) were used as study subjects. Behavioral experiments were used to assess the effects of CB1R on ASD-like behavior in mice, neuronal structural integrity and dendritic density were detected by microtubule-associated protein 2 (MAP2) staining experiments, and the expression of synapse-associated proteins was detected by Western blot, to assess the effects of CB1R on synaptic plasticity. Results Behavioral result showed that VPA mice demonstrated significant ASD-like behavior, while CB1R^{-/ -} mice spent a significantly smaller proportion of residence time in the central region of the open field (P < 0. 0001), showed significant increases in the number of marbles buried and self-grooming time (P<0. 01), significantly less time spent socializing with unfamiliar mice 2 and exploring unfamiliar objects (P<0. 001), and significantly more time exploring old objects (P<0. 05). The relative dwelling time was significantly reduced in CB1R^{+/ -} mice (P<0. 001), and the number of marbles buried and self-grooming time were significantly increased (P<0. 05). Synaptic plasticity assays revealed significant synaptic plasticity impairment in VPA mice. Hippocampal MAP2-positive neuron densities were significantly reduced in CB1R^{-/ -} and CB1R^{+/ -} mice, and expression levels of synapsin-1 were significantly increased (P<0. 05). Conclusions CB1R KO leads to ASD-like behavior such as anxiety and repetitive stereotyped behavior,social and cognitive impairments, as well as neuronal damage, dendritic dysplasia and disrupted synaptic protein expression in mice, suggesting that CB1R is involved in regulating synaptic plasticity as a pathological mechanism for the development of ASD-like behavior.