Varved lake sediment has become one of the important archives for paleoclimate and paleoenvironment studies due to its high-resolution at annual to seasonal timescale. X-ray fluorescence (XRF) core scanning plays an important role in the studies on varved lake sediments due to its high resolution and rapid analysis. In this paper, XRF core scanning was used to study the chronology and formation mechanism of varved lake sediments from Lake Xinluhai in the southeastern margin of the Qinghai-Tibetan Plateau, and the paleoclimate reconstruction based on varve thickness. The results show that varve counting produced using X-radiography, Rad peak, Zr and Fe peak method are essentially consistent with that using the independent radioactive dating (²¹⁰Pb/¹³⁷Cs), which confirms the feasibility and reliability of the above varve counting methods. The annual laminae in Lake Xinluhai were composed of coarse and fine clastic layers. The thick coarse layer with high Zr and Si contents was formed in spring and summer, while the thin fine layer with high Fe content was formed in autumn and winter. The varve thickness can be used to indicate the precipitation intensity of the Southwest Summer Monsoon. The evolution of Southwest Summer Monsoon reflected by the varve thickness in the past 100 years was basically consistent with that of monsoon precipitation reconstructed by humidity (drought/flood index) of Kunming, snow accumulation data from Dasuopu Glacier and the δ¹⁸O of tree ring in the Himalayan region of Nepal. In the past 100 years, the varve thickness of Lake Xinluhai had periodicities of 7-8, 4-5 and 2 years, which may be related with El Niño-Southern Oscillation, Pacific Decadal Oscillation, Indian Ocean dipole and Quasi-biennial Oscillation.