Xinpusaurus is one of the thalattosaurs found in southwestern China. It was first discovered in the Carnian Xiaowa Formation, Guanling County, Guizhou Province. Although abundant Xinpusaurus materials are known, the morphology of juvenile Xinpusaurus and its ontogeny have not been well studied. Here we report a well- preserved and mostly articulated juvenile skeleton of Xinpusaurus suni from Yaoshantian, Xinpu, Guanling County, Guizhou Province. The new specimen is nearly completely preserved, being the smallest reported Xinpusaurus suni individual so far. Methods: Morphological observation and quantitative morphological analysis were conducted to understand the ontogeny of Xinpusaurus suni. The snout/skull ratios of all available Xinpusaurus suni specimens were analyzed linearly and shown by scatter plot. Results: The body length of the new specimen is less than half of the holotype, the skull length is no more than one- third of the holotype skull, the ratio of the skull/trunk length is large, and the sutures of the skull are clear. These lines of morphological evidence support that the new specimen is a juvenile individual. The statistical analysis of all available Xinpusaurus suni specimens reveals that the allometric growth of the snout is significant during the ontogeny. Conclusions: The new specimen supplements the known morphological information of juvenile Xinpusaurus suni. It differs from the adult Xinpusaurus suni by its anterolateral processes longer than anteromedial processes of the supratemporal, pterygoid without teeth, neural arches and centra of the dorsal vertebrae unfused, about 72 caudal vertebrae, distal clavicle significantly shrunk, coracoid nearly round with coracoid foramen, pentagonal ischium, proximal and distal articulation surface of ulna convex. The allometric growth of the snout suggests that the proportion of snout/skull shall not be used to differentiate species in Xinpusaurus.