【Objective】This study aimed to investigate the taxonomy, biology, occurrence and herbivory damage of a new leaf-mining moth feeding on Populus szechuanica var. tibetica so as to provide a reference for its control. 【Method】 A total of 131 plots were set up in 12 districts or counties of Tibet to obtain biological, occurrence and herbivory data. Damaged leaves containing larvae were collected and reared to obtain adult specimens. Pupal exuviae and leaf mines were preserved, and adult genitalia were dissected and mounted on slides for taxonomic identification. 【Result】 According to the morphological characteristics of the leaf mines, pupal exuviae, the adults and the genitalia of the moth, the newly discovered pest was identified as Phyllonorycter pastorella (Zeller, 1846). The morphological diagnostic characters lie largely on the male genitalia, including a narrow valva at the base and a dilated costa at the middle covering short setae, which then remarkably narrows at basal 3/4, with a line of short setae on dorsum, and a horn-like process located ventro-apically. The combined characteristics of the male genitalia, host plants, and leaf mines can facilitate the precise identification of this species. Larvae of this species are leaf miners, making tent-formed mines typical of Phyllonorycter. Among the 131 plots, 29.0% were severely damaged by Phyllonorycter pastorella, distributed throughout Qamdo, Luolong, Nimu, and other counties. Moderate damage accounted for 22.9% and lighter damage accounted for 48.1%, distributed throughout Nanmulin, Jiangda, Chaya, and other counties. Of these, Chaya County has suffered less damage but has seen damage in many more areas. 【Conclusion】 Phyllonorycter pastorella is a newly recorded pest that seriously endangers Tibetan poplar in Tibet, which is a new host plant for this species. Phyllonorycter pastorella should be paid special attention and closely monitored for the prevention and control of pest outbreaks.