Xylotrechus namanganensis Heyden is an important wood-boring pest of Populus euphratica Oliv, Ulmus pumila L and other broad-leaved trees, and it was included in the "List of Dangerous Pests in Forestry" by the State Forestry Administration in 2013. It is very difficult to classify the instar of the larvae because it is extremely cryptic.In this study, the larvae of X. namanganensis were collected periodically from P. euphratica forests in Ejina at different developmental stages, and five morphological indexes of their body length, body width, head capsule width, distance between antennal rings, prothoracic plate width were selected. Frequency distribution diagrams were used to determine the preliminary larval instars numbers, and Crosby's rules and regression analysis were used to verify the analysis. The results showed that the larvae of X. namanganensis could be divided into six instars. The body width, head capsule width, distance between antennal rings, prothoracic plate width, all of which conformed to Dyar's. There was a significant fitting relationship between the measured values and larval instars numbers. Among them the fit between the width of the anterior thoracic dorsal plate (Y) and the age of the larva (X) was the highest, and the regression equation was Y=0.5192e^0.3782x (r²=0.9667), which was the best indicator for larval instar classifications. This article has certain guiding value for the study of the occurrence pattern and bioecological characteristics of X. namanganensis, the development of control strategies and prediction and forecasting of the occurrence pattern of the pest, and it also provides experience for the protection of rare P. euphratica forests in other regions.