Traditional temperature measurement methods for point heat sources include thermocouples, resistance temperature detectors (RTDs), and infrared thermometers. However, these techniques are often limited by their relatively large size, slow response time, or difficulty in achieving precise measurement in small areas. To address these limitations, a temperature sensor based on a photonic waveguide cross-junction and the thermo-optic effect is designed and developed, enabling non-contact temperature measurement of small-sized point heat sources. Photosensitive resin junctions with various morphologies are fabricated using 3D printing. A laser beam is directed at the cross-junction, which disperses the light into its branched structures. An alumina ceramic heating plate serves as the point heat source, providing a small-range, constant temperature, with adjustable temperature control via a variable power supply. The heat emitted from the source alters the refractive index of the micro-junction material (i.e., the thermo-optic effect), thereby changing the intensity of the light emitted from the branches. The change in light intensity is detected in real-time using a photodiode. Experimental results confirm the sensor′s temperature sensitivity and linear response characteristics. The results show that the sensor exhibits excellent measurement accuracy within a small temperature range (330℃~554℃), with good stability and repeatability. In the temperature range of 465℃~554℃, the sensor shows a strong linear relationship, with a detection sensitivity of -9.4 mV/℃, and the repeatability error (δR) ranges from 1.41% to 2.11%. This scheme presents a novel method for temperature detection of micro-nano point heat sources with a simple structure and low cost, providing a new approach for the design of miniaturized, highly sensitive, and non-contact temperature sensors. The methodology exhibits substantial application potential specifically in temperature monitoring for healthcare, laser processing, and 3D printing.