The temperature has a significant impact on the operation, inspection and repair for fuel assemblies in nuclear power plant. This paper introduces a novel technique for monitoring the temperature of spent fuel pool. A piezoelectric probe and a block with parallel inner reflections are used to generate and receive ultrasonic signals in this measuring device. After successfully addressing the challenge of signal processing using FPGA, the ultrasonic transit time difference can be measured with high-precision. This allows for the accurate calculation of ultrasonic speed. Furthermore, by calibrating the relationship between ultrasonic speed and temperature, the temperature can be accurately calculated. Using a heating rod to simulate fuel rod conditions, this approach is capable of measuring the ultrasonic transit time at a resolution of 0.01 ns and temperature measurement resolution of 0.01 ℃. It enables precise measurement in close proximity to fuel assemblies and monitoring the unsteady temperature field due to the residual heat of fuel assemblies.