A large number of random blocks mainly formed by the combination of discontinuities and slope surface can be found in the jointed rock slopes. According to the combination between discontinuities and slope surface, the random blocks are divided into three types block sliding through double sides on slope shoulder, block sliding through double sides on slope surface and block sliding through single side on slope surface. The method of “field qualitative judgment plus stereographic projection” is used to identify the type of random blocks. On the basis of detailed field sampling of discontinuity, three-dimensional discontinuity network simulation is performed by using geological statistics and computer technology. On the basis of former researches on obtaining the diagram of discontinuity trace lines, identifying the largest region where random blocks appear and the combination of points, a method of matching the trace line triangles with the type of the random blocks to indentify the random blocks is put forward. The method is proved to be reasonable by engineering practice. The research provides a new approach to describe and rapidly identify the random blocks of jointed rock slope.