The investigation of rock slope is easier than that of soil, because geotechnical structure is not that complex. But for the consequent rock slope with weak interlayer, the determination of shear strength indexes of weak structural plane has been difficult to grasp. Combined with the engineering case, according to the comprehensive evaluation by direct shear test, standard lookup method and inverse algorithm, shear strength indexes of weak structure of rock slope were determined, and detail introduction and analysis were made on each method to be reference to similar engineering.