In this study, a new species, Nematus pumila Liu, Li & Wei, sp. nov. (Hymenoptera:Tenthredinidae:Nematinae), from China is described. The larvae feed on leaves of Ulmus pumila and even can defoliate an entire tree with high densities, and recurrent defoliation can cause major dieback. This work laid a foundation for identification, monitoring and control of the forest pest. The new species is very similar to N. sheni Wei, 1999, but can be distinguished from the latter by the following characters: labrum, pronotum and tegula of the new species are entirely black; fore tibia, basal 3/4 of middle tibia and basal 3/5 of hind tibia have white maculae; The ratio of width to length of postocellar area is approximately 1.9; fore wing has no Rs vein; petiole of hind anal cell is 1.2 times as long as cu-a; lancet with 15 annuli, serrulae with proximal part distinct, distal teeth big and strong; 1st-11th sutures with ctenidia, the longest ctenidium of lancet is 2/3 length of annulus; radix 0.7 times as long as lamnium. N. sheni Wei, 1999 is recognized as follow:labrum yellowish white; the posterior margin of pronotum, basal margin of tegula yellowish brown; greater part of the ventral side and basal 1/4 of the dorsal side of fore and middle tibiae, basal 1/3 of hind tibia yellowish white; postocellar area approximately 3.0 times broader than long; vein Rs of fore wing vestigial; lancet with 23 annuli, serrulae with proximal part indistinct, distal teeth small; 2nd-15th sutures with ctenidia, the longest ctenidium of lancet is 1/3 length of annulus; radix 0.4 times as long as lamnium. The genomic DNA of the new species was extracted by using the DNeasy Tissue Kit (Qiagen, Valencia, CA), and the sequence data of the mitochondrial gene cytochrome oxidase (COI) were obtained by sequencing, and the length was 810 bp; another 10 COI sequences of known Nematus, Pristiphora, Stauronematus species are available in GenBank and used in phylogenetic analyses with that of the new species. The results of nucleotide composition of the all 12 COI sequences showed that the content of A+T was 65% and higher than that of G+C. Phylogenetic tree of Nematus and related genera species based on Bayesian phylogenetic analysis of COI sequences was constructed, and the results showed that all the Nematus species form a monophyletic group, and Nematus is the sister group of Pristiphora. The holotype and all paratypes of the new species are deposited in the Insect Collection of Central South University of Forestry and Technology, Changsha, Hunan, China (CSCS).