The sensitivity of optical temperature sensing based on the conventional rare-earth ion doped upconversion (UC) materials is limited by the energy gap between thermally coupled levels (TCLs) of rare-earth ions. Therefore, it is of great theoretical and technical interest to explore UC luminescent materials for optical temperature sensing with ultra-sensitive temperature characteristic. In this work, the UC luminescence properties and temperature sensing characteristics were studied for Er ³⁺ single-doped BiOCl excited by 1550 nm laser. Under near-infrared (NIR) excitation, BiOCl:Er ³⁺ exhibits strong red emission at 670 nm, weak green emissions at 525 and 542 nm, extremely weak violet emission at 406 nm, and near-infrared emission at 983 nm. Red and green emissions of the UC system exhibit strong temperature dependence, and in the temperature range of 300-563 K, the maximum absolute sensitivity (S_A) obtained by employing the non-thermally coupled levels (NTCLs, ⁴F_9/2/ ⁴S_3/2) is 95.3×10 ^-3K ^-1, which is 21 times more than that obtained by employing the thermally coupled levels (TCLs, ²H_11/2/ ⁴S_3/2), and the maximum relative sensitivity (S_R) is as high as 1.19% K ^-1. The results show that the intense red UC luminescence and temperature sensing with ultra-high sensitivity in BiOCl:Er ³⁺ under 1550 nm excitation may have potential application prospect in display and optical temperature sensing.