Abstract: As a product of collision orogenic belt, leucogranites are not only closely related to rare metal mineralization, but also have great significance to orogenesis and plateau uplift mechanism. The petrogenesis of leucogranites is still controversial. Earlier studies thought that leucogranites were generated by low-degree in situ partial melting of metasedimentary rocks, but in recent years, more scholars have pointed out that they may be highly fractionated granites, and the magma had undergone an intensive fractional crystallization. In order to explore the petrogenesis of leucogranites, we collected Lalong leucogranites and their sedimentary surrounding rocks (marbles and slates) in the eastern Himalayan orogenic belt to analyze the major and trace elements, Sr-Nd isotopes and monazite U-Th-Pb dating. Lalong leucogranites are located in the core of the Lalong dome, which two-mica granites, muscovite granites and albite granites are exposed in turn from inside to outside. The dating results show that these three leucogranites have similar emplacement ages (22~23 Ma). Generally, the leucogranites show high SiO₂(73.0%~75.7%), high K₂O (3.50%~6.53%), low MgO (0.03%~0.22%) and peraluminum (A/CNK=1.05~1.24). From two-mica granites through muscovite granites to albite granites, the negative Eu anomalies becoming intensified, Rb, Rb/Sr and Y/Ho gradually increased, while Sr, Ba, K/Rb and Zr/Hf gradually decreased. In addition, these leucogranites have consistent Sr-Nd isotopic compositions: (⁸⁷Sr/⁸⁶Sr)_i=0.736456~0.737929, ε_Nd(t)=-12.4~-12.1, which are depleted than that of the surrounding rocks (ε_Nd(t)=-16.9~-15.1). Two-mica granites have consistent Sr-Nd isotopic compositions with the Higher Himalayan Crystallines, the high CaO/Na₂O ratio values (0.33~0.42) and Al₂O₃/TiO₂ ratio values (197~459) indicate that their source region is dominated by clastic rocks. The spatial distribution characteristics in the field and data results show that the Lalong leucogranites may have originated from muscovite dehydration melting of metagreywacks in Higher Himalayan Crystallines, which had undergone certain degree of fractional crystallization rather than generating by in situ partial melting of metasedimentary rocks. The trace elements Rayleigh fractionation modeling results also proved that when two-mica granite is used as the initial melt, it could generate muscovite granites and albite granites after fractional crystallization of ~70% and ~90%, respectively. Considering the overlapping relationship between the South Tibetan Detachment System and the Lalong leucogranites, this paper considers that the South Tibetan Detachment System may have triggered partial melting of the leucogranites source region by decompression and provided space for the flowage differentiation of the leucogranites.