Abstract: In order to clarify the strength deterioration characteristics and microscopic mechanism of  Qinghai-Xizang Plateau moraines during freeze-thaw cycles, laboratory tests (standard triaxial, scanning electron microscopy and nuclear magnetic resonance tests) and numerical regression methods were used to study the moraines under different initial conditions (water content and freeze-thaw cycles). A quantitative relationship between macroscopic strength parameters and freeze-thaw action was established through regression analysis. The research results show that: 1) The stress-strain curve of the moraines after the freeze-thaw cycle exhibited a strain softening trend, with both elastic modulus and shear strength showing a downward trend, in the early freeze-thaw cycle (0-5 times) damage by about 16.60% and 10.33% respectively, strength degradation is most obvious, and later in the freeze-thaw cycle (10-20 times) a slight fluctuations and steady state. Freezing and thawing cycles and cohesion of negative exponent, internal friction angle with the increase of freeze-thaw cycles was fluctuated, extends the applicable under the ice moraine soil freezing and thawing cycle of Mohr coulomb strength criterion, and established the shear strength, elastic modulus and multi-parameter coupling mathematical model of freeze-thaw cycles. 2) SEM images showed that with the increase of freeze-thaw cycles, the arrangement of moraine particles changed from dense and ordered to loose and disordered.And the pore characteristic structure changes. 3) NMR results demonstrated evolution characteristics in the internal pore structure of moraines under the freeze-thaw cycle. With deepening freeze-thaw cycles, large, medium pore proportion increased to 59.55%（N=20） micro, small pore proportion reduced to 40.45%（N=20）. The small and small pores gradually expanded and merged under the action of frost heave force generated during the water-ice phase transition. Large and medium pores were formed, and the characteristic evolution process of pore structure was basically completed in the late freeze-thaw period (10-20 times).

Key words: Qinghai-Xizang plateau；moraine soil')">

Qinghai-Xizang plateau；moraine soil, freeze-thaw cycles, strength deterioration；microscopic mechanism;elastic modulus；shear strength