The　mechanical　properties　and　deformation　behavior　of　soil-rock　mixture　and　underground　structure　in　soil-rock　mixture　stratum　are　influenced　by　the　particle　size　distribution.　The　mesoscopic　structural　characteristics　of　soil-rock　mixture　were　analyzed　based　on　the　fractal　geometry　theory　and　the　relationship　between　the　quality-particle　size　grading　curve　and　the　fractal　dimension　was　determined.　The　fractal　analysis　on　grain-size　of　34　groups　of　sand-gravel　soil　in　coordinate　system　lg(M(rT)lgR　was　conducted　and　the　results　indicated　that　the　sand-gravel　in　Beijing　has　the　characteristics　of　self-similarity.　The　range　of　the　fractal　dimension　of　sand-gravel　in　Beijing　is　2.4-2.6.　The　variation　of　the　quality-particle　size　grading　curve　with　the　fractal　dimension　and　maximum　grain　size　were　analyzed,　yielding　some　new　findings　regarding　the　mechanical　behavior　of　soil-rock　mixture.　It　is　clear　that　the　whole　grading　curve　can　be　obtained　by　combining　the　fractal　geometry　theory　and　the　laboratory　test　on　grain　size　distribution.　A　program　generating　the　model　of　sand-gravel　stratum　is　compiled　based　on　the　Monte　Carlo　method　using　the　sphere　to　represent　the　shape　of　stone.　Then,　3D　numerical　analysis　on　tunnel　excavation　were　carried　out.　©　2017,　Science　Press.　All　right　reserved.