For researching the cavability of No. 92 ore body in Tongkeng Mine, quit a number of investigation of engineering geology has been carried out. Making use of equatorial projection and computer aid design technology to find out triple dominant position joint sets, qualitative classification of roof rock mass is carried out according to the rock mass rating (RMR) and Q Classification systems that are very often used in the world at present, and the result is the third grade. In addition, according to Laubscher's geo-mechanics Classification system of rock, which has been properly modified by mining influence coefficient, the mining rock-mass rating (MRMR) has been evaluated, and it is the fourth grade. At last, the reasonableness of primarily designed caving area is evaluated by the MRMR and Laubscher's caving graph. The results are significant for the engineering practice and other similar projects.