Lead halide perovskites have been very promising for versatile optoelectronic applications, whereas the inherent toxicity and instability of lead halide restrict its wide application. Herein, a kind of high‐performance multifunctional phosphor is designed based on heavy concentration Er3+‐activated lead‐free double perovskite Cs2NaBiCl6 (CNBC), which realizes the self‐sensitization of Er3+ ions and emits high‐brightness pure green emission upon dual near‐infrared (dual‐NIR) excitation. The luminescence properties can be further optimized by adding sensitizers (Yb3+/Nd3+). Interestingly, the emission intensity of the green region is heightened by 212‐fold after Yb3+ ions doping, which is 40 times higher than that of the current popular phosphors (α‐NaYF4:2%Er3+,18%Yb3+). Meanwhile, the temperature sensing properties of the thermally coupled levels (Er3+:2H11/2,4S3/2) in Cs2NaBiCl6:40%Er3+ are also investigated systematically based on the luminescence intensity ratio principle, and the maximum relative sensitivity is calculated to be as high as 1.27% K‐1 (980 nm) and 1.57% K‐1 (808 nm). Fortunately, the thermal stability and the performance for temperature readout of the studied phosphors are still maintaining a high level after doping sensitizers. The insights provided by this work will broaden the scope of lead‐free halide double perovskites in the fields of luminescence and thermometry.

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