Completely decomposed migmatitic granite（CDMG）is a kind of rock with special genesis

but there is little research on its mechanical properties and microstructure. In order to explore the relationship between macroscopic strength characteristics and microstructure of Lincang CDMG

triaxial tests and scanning electron microscopy tests were carried out on samples with different moisture contents

and the microstructure parameters that characterized the size

morphology and orientation of particles and pores were extracted and calculated. By investigating these microstructure parameters

the micro mechanism controlling the macroscopic strength characteristics of CDMG is revealed. The results show that with the increase of moisture content

the stress-strain curve of the sample exhibits a dissimilar hardening effect

the shear strength deteriorates significantly

the internal friction angle decreases linearly

and the cohesion fluctuates up and down. Under the condition of low moisture content

the samples are mainly characterized by micro and small pores

and the particle morphology is mostly angular and sub-angular strip with certain orientation. With the increase of moisture content

the pore content shows an upward trend

forming evenly distributed honeycomb medium and large pores

which are easier to be compressed under axial load. The shape of particles develops to be round

and the overall distribution is disorderly and poorly oriented. Comprehensive analysis of the above results shows that the mechanical properties of CDMG are essentially the result of the interaction between microscopic particles and pore structure. The increase of moisture content weakens the friction between coarse particles

resulting in uneven expansion of clay particles

which leads to pore connection and coarsening and damage of filling structure at the micro level

and then shows mechanical properties reduction at the macro level. The research results provide useful reference for the cognition of mechanical properties and microstructure damage evolution of CDMG.