Abstract: Mercury slag accumulation are widely distributed on natural slopes in Guizhou province of China，which are of great danger and harmfulness，so it is in urgent need to study its shear strength characteristics and mechanism.Mercury slag has the characteristics of loose structure，poor sorting，different particle size and weak cementation.A typical site of Danzhai county，Guizhou province was selected to carry out the mercury slag particle analysis test and the field large-scale direct shear test.Based on the particle discrete element PFC2D program，the numerical tests corresponding to the field direct shear test were carried out，and the meso mechanism of shear resistance was studied.The curves and shear strength parameters obtained by field test and numerical test are close to each other，which proves the reliability and high fidelity of the numerical test.Test results show that：When the shear stress is near the peak，the obvious tensile force appears near the shear zone，but the tensile force is not obvious in the far zone.As the shear proceeds，the particles of the active shear plate move to the right as a whole;while the particles of the fixed plate near the shear plate and the shear band also move towards the right.There are several stress concentration zones near the shear zone，and the stress is larger as it closes to the shear wall.Large particles have great influences on stress distribution.The stress concentration zone in the fixed plate is close to the shear band，and the stress concentration zone of the active shear plate is more widely distributed and more greatly affected by the shear action.

Key words: Mercury slag, Direct shear test, Shear strength, Meso mechanism, PFC