Abstract: The volcanic rocks of Xinmin Formation in Sangenhe forest farm are mainly composed of rhyolites and rhyolitic ignimbrites. The U-Pb dating results indicate the rhyolites were formed in Middel Jurassic, with the age of 165.71±0.83 Ma. The petrogeochemical analysis shows that the rhyolites are characterized by relatively high silicon, alkali and peraluminous, with the SiO₂ content of 71.05%-76.77%, Na₂O+K₂O of 6.66%-8.49% and A/CNK of 1.02-1.17. The rocks present medium total REE content, significant fractionation of HREEs and LREEs, strong negative Eu anomaly (δEu=0.39-0.66) and negative Dy anomaly, relative depletion of LILEs (Ba and Sr), enrichment of HSFEs (Th and U) and relative depletion of Nb, Dy and Ti, with the characteristics of A-type rhyolites. The study shows the magma is derived from partial melting of crustal materials. The Xinmin Formation is spatially distributed along the middle-north section of Daxinganling volcanic belt, revealing the volcanic belt was in a tensional tectonic environment in Middle Jurassic.