The development of highly efficient, stable, and color-tunable lead-free perovskite phosphors is a central challenge for their application in next-generation optoelectronic devices. In this work, a series of Cs₂NaGd_0.985Cl₆:0.015Sb³⁺ phosphors with different Er³⁺ concentrations are successfully synthesized using a microwave-assisted solid-state method. The X-ray diffraction results confirm that the introduction of Er³⁺ does not cause any crystal structure change or impurity phase formation. Under 336 nm excitation, the material exhibits a broad blue emission centered at 460 nm from self-trapped excitons (STEs) of the host, as well as characteristic green/red emissions of Er³⁺ ions (524 nm, 550 nm, 667 nm). By investigating the concentration-dependent luminescence behavior, the optimal Er³⁺ doping concentration is determined to be 0.03, resulting in maximum emission intensity with an absolute photoluminescence quantum yield of 37.09%. The concentration quenching mechanism is attributed to electric dipole-dipole interaction. At this optimal concentration, , steady-state and transient fluorescence spectroscopy analyses confirm the existence of an efficient energy transfer channel from the host STEs to Er³⁺ ions, with a calculated energy transfer efficiency of 24.58%. This process significantly enhances the characteristic emission of Er³⁺ and is key to achieving efficient multicolor luminescence. Furthermore, the optimized sample Cs₂NaGd_0.955Cl₆:0.015Sb³⁺, 0.03Er³⁺ at 423 K demonstrates excellent thermal stability, retaining 69.4% of its room-temperature (298 K) emission intensity. More importantly, tunable luminescence from blue (CIE: 0.160, 0.194) to green (CIE: 0.215, 0.374) is successfully achieved by simply adjusting the Er³⁺ concentration. This work not only deeply elucidates the energy transfer pathway and concentration quenching mechanism in Sb³⁺/Er³⁺ co-doped double perovskite system from the perspective of physical mechanism, but also experimentally demonstrates that the developed lead-free phosphor, which integrates high quantum efficiency, excellent thermal stability, and broad color tunability into one material, has broad application potential as core luminescent material in high-performance, environmentally friendly green light-emitting diodes.