The　distribution　curve　of　the　over-excavation　and　under-excavation　of　sap　outline　was　analyzed.　It　was　found　that　the　curve　was　intuitively　self-semblable.　A　lot　of　data　of　the　over-excavation　and　the　under-excavation　of　sap　outline　were　collected.　Based　on　engineering　rock-mass　quality　classification　standard,　the　rock-mass　quality　of　the　above　mentioned　sap　was　divided　into　three　types,　namely,　Types　II,　III　and　IV.　The　collected　data　were　also　divided　into　three　groups.　The　double　logarithm　sketch　of　each　group＇s　number　of　data　and　their　values　was　drawn.　After　the　linear　regression　equation　of　each　data　group　was　figured　out　through　the　least　double　multiplication,　it　was　found　that　the　linearity　correlation　degree　was　very　high.　Therefore,　the　distribution　of　the　over-excavation　and　under-excavation　value　of　the　saps　outline　located　in　Types　II,　III　and　IV　rock　mass　conformed　to　fractal　distribution.　And　the　fractal　dimension　of　the　curve　of　each　group　of　data　distribution　was　calculated.　It　was　found　that　the　fractal　dimension　of　the　curve　of　over-excavation　and　under-excavation　value　increased　when　the　quality　of　surrounding　rock　get　poorer　under　certain　conditions.　A　foundation　for　the　analysis　of　the　stability　of　tunnel　structures　and　the　random　research　model　of　the　stability　of　tunnel　structures　was　established　with　the　above-mentioned　results.