The Long'an deposit is the only mining ion-adsorption type REE (iREE) deposit in Yunnan Province, the parent rock of which is the Banggunjianshan (BGJ) granite. Recently, comparable systems have been recognized in the south-middle part of weathering BGJ granite. What's more, some of these deposits are relatively heavy REE (HREE) enrichment. In order to investigate the controlling factors of iREE deposit, the south part of BGJ granite has been chosen to study the petrology, mineralogy, geochemistry and chronology. The host rock of BGJ granite is biotite monzogranite, zircon U-Pb date of which is 50.33 Ma (50.33±0.30 Ma, MSWD=0.15), indicating that it was formed in Paleogene. It's the youngest age of parent rock of discovered iREE deposits. BGJ granite with high A/CNK ratios (＞1.1), are strongly potassium rich and display peraluminous characteristics similar to S-type granite. BGJ granite is relatively LREE enriched, has strongly fractionated REE patterns (LREE/HREE=2.54~8.98), moderately negative Eu anomalies (Eu/Eu*=0.06~0.17), is high in Rb, Th, and depleted in Ba, Nb, Sr, Zr, Hf, Ti. REE bearing minerals (REE minerals) are the key factor for REE concentration in the rock. The REE minerals in BGJ granite are sphene (∑REE=14506.24×10^-6, LREE/HREE=1.14, n =6), allanite (∑REE=232860.82×10^-6, n =2), fergusonite, zircon, thorite, fluorite, REE-fluocarbonate; however, apatite, monazite and xenotime are not uncommon. In addition, biotite (∑REE=168.60×10^-6, LREE/HREE=1.41, n =10) and feldspar (∑REE=123.60×10^-6, LREE/HREE=2.77, n =8) also contribute to REE concentration in the BGJ granite. The REE bearing minerals dissolve in the weathering crust and release REE that could be the source of iREE deposits. Through this study, the following conclusions are obtained: ① REE migrate, fractionate and re-enrich because of the evolution of granite with little change in REE content; ② During granite evolution, REE moved out from the original minerals, subsequently forming secondary REE minerals that are the key to forming iREE (even iHREE) deposits; ③ In bedrock rocks (BGJ granite) of the Long'an iREE deposit, sphene, allanite, thorite, fluorite and REE-fluocarbonate minerals are the main sources of iREE mineralization, followed by feldspar and biotite. These minerals show that relative LREE enrichment led to the formation of iLREE deposit.