The Boluositanmiao Complex in the eastern part of the Alxa Block, is one of the high-grade metamorphic complexes, which has important geological significance for the late Paleozoic tectonic evolution of the Alxa Block. In this paper, a combined study of detailed geological investigation, petrography, LA-ICP-MS monazite U-Pb dating and phase equilibrium modeling, were carried out for felsic gneisses of the Boluositanmiao Complex, which is used to reveal their metamorphic evolution and tectonic implications. Felsic gneisses are mainly composed of garnet-bearing biotite-plagioclase gneiss and biotite-plagioclase gneiss. The garnet-bearing biotite-plagioclase gneiss samples ALS2202 and ALS2204 record peak mineral assemblage of garnet+biotite+muscovite+ilmenite+plagioclase+K-feldspar+quartz+melt. Combined with phase equilibrium modeling, the peak p-t conditions of two samples were constrained at 727~751℃/820~1 030 MPa and 730~751℃/930~1 070 MPa, respectively. In addition, the garnet-biotite-plagioclase-quartz (GBPQ) geothermobarometry yields relatively low metamorphic p-t conditions of 629~672℃/410~490 MPa and 620~657℃/330~400 MPa, which correspond to the retrograde cooling. Based on the above results, a clockwise p-t evolution was obtained for the garnet-bearing biotite-plagioclase gneiss samples ALS2202 and ALS2204. LA-ICP-MS monazite U-Pb analyses obtained the weighted mean ²⁰⁶Pb/²³⁸U ages of 268±4 Ma and 255±2 Ma for the garnet-bearing biotite-plagioclase gneiss sample ALS2207 and the biotite-plagioclase gneiss sample ALS2210, respectively, which may represent the timing of peak metamorphism or anatexis. Based on the mineral assemblage, it suggests that the protolith of these felsic gneisses should be sedimentary rocks. Combined with the clockwise p-t path and previously published data in the region, this indicates a significant crustal thickening process in the middle and late Permian, which may be related to the closure of the Paleo-Asian Ocean.