We carry out the first theoretical investigation on yields and the hardest parton splitting of large-radius jets reclustered from small radius ( R = 0.2 ) anti- k t jets in Pb + Pb collisions, and confront them with the recent ATLAS measurements. The Linear Boltzmann Transport (LBT) model is employed for jet propagation and jet-induced medium excitation in the hot-dense medium. We demonstrate that, with their complex structures, the medium suppression of the reclustered large radius jets at R = 1 is larger than that of inclusive R = 0.4 jets defined conventionally. The large radius jet constituents are reclustered with the k t algorithm to obtain the splitting scale d 12 , which characterizes the transverse momentum scale for the hardest splitting in the jet. The large-radius jet production as a function of the splitting scale d 12 of the hardest parton splitting is overall suppressed in Pb + Pb relative to p + p collisions due to the reduction of jets yields. A detailed analyses show that the alterations of jet substructures in Pb + Pb also make significant contribution to the splitting scale d 12 dependence of the nuclear modification factor R AA . Numerical results for the medium modifications of the jet splitting angle Δ R 12 and the splitting fraction z are also presented.