Lead-free double perovskites have attracted noteworthy attention due to their compositional flexibility and electronic diversity. In this study, we hydrothermally grow a new class of Cs2AgxNa1-xFeCl6 (0 ≤ x ≤ 1) perovskite single crystals with high thermal stability. The substitution of B-site cation allows to regulate the crystallographic and band structure, which gives rise to enlarged band absorbance close to the near-infrared region (∼800 nm) via composition engineering. Ultrafast transient absorption spectroscopy (TAS) certifies that the decay time of excited-state absorption is 5.02 and 2450 ps in the case of Cs2NaFeCl6 and Cs2AgFeCl6, respectively. The corresponding charge carrier diffusion length accordingly enhances from 3.7 to 311 nm by means of increasing Ag dopant concentration. Structurally, the primitive cell shrinks due to the partial replacement of [NaCl6]5- octahedra by [AgCl6]5- octahedra. It is proved theoretically as well as experimentally that the introduction of Ag species can effectively enhance the electron mobility (from 1.06 to 15.3 cm2 V-1 s-1) by ∼15 times through realizing stronger orbital coupling of the conductive ions, which enables such a novel double perovskite to be a potential candidate for the optoelectronic and photovoltaic applications.