Lead-free double perovskites have recently received increasing attention due to their variable band gap. In this work, tin was doped into CsBiAgClsuccessfully. The doping of Sncations can make the weak white luminescent host to produce pure blue emission peaking at around 464 nm. The internal carrier changes after Sndoping and the origin of blue emission in CsBiAgCl:Snwas analyzed by ultrafast transient absorption (TA) techniques and time-resolved spectra. To further study the substitution feasibility of Sn-Snfor Bi-Bipair cations, Rietveld refinement and density functional theory (DFT) calculations were performed, which proved that the crystal structures of CsBiAgCl:Sncan exist stably. Based on these observations, we introduced blue-emitting CsBiAgCl:Snpowder into a white LED fabricated by combining a 340 nm chip to generate stable white light with CIE coordinates of (0.31, 0.34) and a color rendering index value of 83. This work provides the methods to develop environmentally friendly blue emission from lead-free double perovskites at low cost.