The development of luminescent probes in biological windows has emerged as an exciting field by virtue of nanoscale spatial resolution and low tissue scattering and absorption. However, current luminescent nanothermometers in the biological windows still suffer from limited relative temperature sensitivity (S_r). Herein, one ultrasensitive ratiometric nanothermometer in the first biological window is designed based on NaYb(MoO₄)₂:Tm³⁺ nanosheets sensitized by Yb³⁺–MoO₄²⁻ dimers. Due to the cooperation of phonon-assisted energy transfer and thermally coupled population, the 691 nm emission of Tm³⁺ ions realizes enhancement to 48-fold at 573 K, while the 651 nm emission of Tm³⁺ ions first increases slightly and then quenches severely because of the competition between phonon-assisted thermal enhancing and thermal quenching. These high-contrast thermal-dependent characteristics create a new S_r record of 6.5% K⁻¹ at 313 K in the biological windows with high repeatability (better than 98.3%) and low temperature uncertainty (0.16 K). After tuning the luminescence process by Yb³⁺–MoO₄²⁻ dimers, the power-dependent thermometric signals are determined by constructing calibration curves. Further the temperature detection of biological tissues confirms the feasibility of the proposed nanothermometer.