Non-contact temperature sensors with low cost, high reliability and high sensitivity have attracted increasing research interest in recent years. In this study, we synthesized a bimodal optical temperature sensor Sr2SiO4:Ce3+/Eu2+/K+ with excellent thermometric sensitivity through a high-temperature solid-state reaction method. In the matrix of α-Sr2SiO4, Ce3+ luminescence exhibits excellent thermal stability (∼129.1%@250 °C), while Eu2+ shows strong thermal quenching (∼21.7%@250 °C), leading to a significant change in the fluorescence intensity ratio (FIR) of Ce3+ (437 nm) and Eu2+ (550 nm) as a function of temperature. This feature enables the phosphor exhibiting outstanding sensitivity in the temperature range of 298-523 K. To be exact, it demonstrates a maximal relative sensitivity of 0.93% K-1 at 348 K. Its absolute sensitivity linearly increases and reaches 3.46% K-1 at 523 K. Besides, it has a large chromaticity shift (ΔE = 228 × 10-3 in 298-523 K) against temperature, making the temperature change visible to the naked eye. We first demonstrate a CIE chromaticity coordinate technique for temperature sensing with high accuracy and good sensitivity by using the function of x or (x2 + y2)0.5 against T. These unique optical thermometric features allow Sr2SiO4:Ce3+/Eu2+/K+ to serve as an accurate and reliable thermometer probe candidate for temperature sensing applications.