Magmatic rocks are widely distributed in Beishan area, and the formation age of these intrusive rocks can span from early Paleozoic to Mesozoic. Although many studies have been carried out on the formation age of magmatic rocks in the Beishan area, most of them focus on the Paleozoic magmatic rocks but little on the Mesozoic ones. Laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) zircon U-Pb dating was applied to determine the age of porphyritic monzonitic granite in Hongqishanjing, Beishan, in Gansu Province, which revealed its average age at (237.6 ± 1.1) Ma (MSWD = 0.44) during Middle Triassic. The geochemical analysis shows that the rock is characterized with high silicon (SiO₂ = 67.17% ~ 68.33%), high alumina (Al₂O₃=15.02% ~ 15.82%), poor magnesium (MgO = 1.07% ~ 1.18%), and rich sodium and potassium (Na₂O = 3.35% ~ 3.75%, K₂O = 4.2% ~ 4.38%, Na₂O/K₂O = 0.80% ~ 0.85%). The contents of TiO₂ and CaO are 0.57% ~ 0.64% and 2.39% ~ 2.6%. The LREE/HREE ranged from 36.86 to 44.52, showing obvious enrichment of light rare earth and slightly negative anomaly of Eu. Trace element analysis reveals the enrichment of the large ionic lithophilic elements Ba and strongly incompatible elements Th and U and depletion of the high field strength elements such as Nb, Ta, Ti, Y, Yb and Lu, showing the characteristics of I-type granite. Therefore, combined with the regional tectonic characteristics, it suggests that the Beishan Ocean Basin has not been closed until the Middle Triassic, and the southern Beishan Belt is in the continental margin island arc environment under the subduction of the oceanic plate.