Updated project metadata. ER degradation by selective autophagy (ER-phagy) is crucial for ER homeostasis. However, it remains unclear how ER scission is regulated for subsequent autophagosomal sequestration and lysosomal degradation. Here, we show that oligomer formation of FAM134B (also referred to as reticulophagy regulator 1 or RETREG1) through its Reticulon domain was required for membrane fragmentation in vitro and ER-phagy in vivo. Under ER stress conditions, activated CAMK2B phosphorylated the Reticulon domain of FAM134B, which enhanced FAM134B oligomerization and activity in membrane fragmentation to accommodate high demand of ER-phagy. Unexpectedly, FAM134BG216R, a variant derived from a type II HSAN patient, exhibited gain-of-function defects, such as hyperactive self-association and membrane scission, which resulted in excessive ER-phagy and sensory neuron death. Therefore, this study revealed a mechanism of ER membrane fragmentation in ER-phagy, along with a signaling pathway in regulating ER turnover, and suggested a potential implication of excessive selective autophagy in human diseases.