We have developed a high-precision thermometry strategy based on 4I9/2-4I15/2 (I800 nm) and 4S3/2-4I15/2 (I551 nm) transitions of Er3+, after replacing the measurement of the 4I11/2-4I15/2 (I1000 nm) transition with the 4S3/2-4I15/2 transition, i.e., using visible light for detection instead of infrared. Through rate equation model analysis, (I1000 nm)2 and I551 nm can be substituted for each other under certain conditions. Further, because the 4I9/2 and 4I11/2 levels of Er3+ are thermally coupled, a new idea of ratiometric thermometry is proposed based on the ratio of (I800 nm)2 and I551 nm, which has the advantages of anti-interference of excitation light source fluctuation and background-free detection. The feasibility of the idea was verified by researching the power-dependent emission spectra at different temperatures and temperature-dependent emission spectra of a CaWO4:Er3+,Yb3+ sample under 980-nm laser excitation. The maximum relative sensitivity for the new ratiometric thermometry reaches up to 7.4% K-1 and the optimal temperature uncertainty calculated is 0.03 K at 303 K. This study provides guidance for solving the problem of a weak response of an infrared detector.