Amphibian resources are abundant and unique in China, and more new amphibian species were found through molecular techniques. A Microhyla species was found during a field survey in Zhejiang Jiulongshan National Nature Reserve in April of 2017 and 2018. They were identified based on morphological comparison, BLAST comparison and phylogenetic analysis of mitochondrial 16S rRNA gene. Thirty-one adult (25♂♂ and 6♀♀) were captured and measured for snout-vent length (SVL), head length (HL), head width (HW), snout length (SL), internasal distance (IND), internorbital distance (IOD), diameter of eye (DE), length of lower arm and hand (LLAH), total length of leg (TLL) and foot length (FL) to examine sexual dimorphism of this species. Corresponding data were processed by one-way ANOVA, linear regression and principal component analysis. Our results showed that: (1) the species was identified as M. beilunensis and this sample locality is a new geographic distribution of M. beilunensis (Figure 1); (2) M. beilunensis displays female-biased sexual size dimorphism (Table 3); (2) other nine morphological traits were positively related with SVL in both sexes of M. beilunensis (Figure 3); (3) the mean value of LLAH in males was larger than female after removing the effect of SVL (Table 3); (4) a principal component analysis resolved two components (eigenvalues 1) from nine SVL-adjusted morphological traits, accounting for 52.47%% of variation in the original data (Table 4). The first component had high positive loading for HW and LLAH, and morphological traits between male and female differed significantly in their scores on the first axe (Table 4), indicating that HW and LLAH could distinguish females and males.