On the basis of five prototype top storey T-joints in R.C. frame structure with special-shaped columns, this paper examines shearing characteristic and mechanism. Based on the theory of softened strut-tie, the formulas for shearing strength of top story exterior joints in R.C. frame structure with special-shaped columns are derived. Three calculations are given in this paper, which are theoretical calculation, technical specification calculation and experimental value. The theoretical calculating value is compared with the testing results and the analysis shows that the technical specification formula can ensure high safety.