Myotonic dystrophy type 1 is a multisystemic disorder that has been extensively studied for decades, yet our understanding of its neuropathological aspect remains rudimentary. Building on an established Drosophila model, we study the neuropathological features of the disease by expressing untranslated expanded CUG repeats at the Drosophila larval neuromuscular junction. In this model, we show that both pre- and postsynaptic expressions of CUG repeats participate in inducing phenotypes in synaptic boutons, arbors, transmission and larval locomotor activity. Furthermore, expression of CUG repeats in either motorneurons or body wall muscles induces upregulation of the cell adhesion molecule FasII (NCAM1 in mammals), and the knockdown of fasII is sufficient to rescue the phenotypes. Overexpression of FasII-C, a FasII isoform with no cytoplasmic domain, mimics the phenotypes of expanded CUG expression at the neuromuscular junction. In contrary, overexpression of FasII-A-PEST+ rescues the synaptic and behavioral defects. Our study provides insights into the fundamental mechanisms underlying synapse dysregulation in myotonic dystrophy type 1.