Insufficient upconversion luminescence (UCL) intensity under low-irradiance remains a critical bottleneck of Lanthanide-doped nanoparticles, which severely limits their further application. Herein, through a multi-field enhanced strategy combining photonic crystal, plasmonic, and low-temperature fields, an UCL enhancement factor of 1290 is achieved in an Er³⁺-rich nanosystem. Based on that, a distinctive multimode fluorescence ratiometric upconversion thermometer is well-achieved. Specifically, leveraging the multi-wavelength excitation capability of the Er³⁺-rich system, six independent operating modes based on infrared-to-green emission intensity ratios are realized in a single composite structure composed of upconversion nanoparticles, gold nanorods, and PMMA opal photonic crystals under 808, 980, and 1530 nm excitation. These modes endow the thermometer with flexible operation selection, high sensitivity, and high precision across the 80-300 K range. The maximum relative sensitivity of all the six modes reaches 7.06-14.95% K^-1, with a high resolution in the range 0.008-0.634 K, which is several to tens of times higher than that of the conventional upconversion systems. This work not only provides an effective strategy for substantially enhancing UCL but also offers new insights into the design of high-performance optical thermometers.