Autophagy, identified as type II programmed cell death, has already been known to be involved in the pathophysiology of preeclampsia (PE), which is a gestational disease with high morbidity. The present study aims to investigate the functional role of let‐7i, a miRNA, in trophoblastic autophagy. Placental tissue used in this study was collected from patients with severe preeclampsia (SPE) or normal pregnant women. A decreased level of let‐7i was found in placenta of SPE. In addition, autophagic vacuoles were observed in SPE and the expression of microtubule associated protein 1 light chain 3 (LC3) II/I was elevated. In vitro, let‐7i mimics suppressed the autophagic activities in human HTR‐8/SVneo trophoblast cell line (HTR‐8) and human placental choriocarcinoma cell line JEG‐3, whereas let‐7i inhibitor enhanced the activities. As a potential target of let‐7i, autophagy‐related 4B cysteine peptidase (Atg4B) had an increased expression level in SPE. As expected, the increased expression of Atg4B was negatively regulated by let‐7i using dual luciferase reporter assay. Furthermore, these trophoblast‐like cells transfected with the let‐7i mimic or inhibitors resulted in a significant change of Atg4B in both mRNA and protein level. More importantly, Atg4B overexpression could partly reverse let‐7i mimic‐reduced LC3II/I levels; whereas Atg4B silencing partly attenuated let‐7i inhibitor‐induced the level of LC3II/I expression. Taken together, these findings suggest that let‐7i is able to regulate autophagic activity via regulating Atg4B expression, which might contribute to the pathogenesis of PE. J. Cell. Physiol. 232: 2581–2589, 2017. © 2016 Wiley Periodicals, Inc. This study provided insight into the role of let‐7i in the pathophysiology of PE. Our study demonstrates for the first time that a novel let‐7i‐Atg4B‐autophagy signaling pathway plays a key role in PE. This finding suggests that miRNAs are key molecules involved in autophagic activity, and might be regard as a new therapeutic target against PE.