The near-infrared-II (NIR-II) fluorescence thermometer based on rare earth-doped double perovskite is rarely. Herein, Er3+-Ho3+ and Yb3+ co-doped lead-free double perovskite La2MgTiO6 (LMTO) phosphors were synthesized by a conventional high temperature solid phase method. The phase purity and morphology of the samples were analyzed using XRD and FE-SEM. The NIR-II luminescence properties of LMTO: Er3+-Ho3+-Yb3+ phosphors were systematically investigated under laser irradiation at 980 nm. A NIR-II fluorescence thermometer was also designed based on non-thermal coupled energy levels originated from the 4I13/2 energy level of Er3+ and the 5I6 energy level of Ho3+. The maximum value of relative sensitivity (Sr) was 0.54 % at 513 K. Compared to visible thermometers (LMTO: Er3+-Yb3+ and LMTO: Ho3+-Yb3+), the NIR-II thermometers offer better temperature measurement capability and penetration depth. All the experimental results show that the NIR-II thermometer with a wider range and higher sensitivity has been realized, which puts forward a new idea for the design and optimization of NIR-II thermometers.