Terahertz noise sources are critical tools for noise figure measurement and performance evaluation of high-frequency devices. Traditional solid-state noise sources based on electronics face challenges in achieving high excess noise ratio (ENR) and flat power spectral characteristics due to the bandwidth limitations of electronic components, restricting their application in higher frequency bands. To address this issue, this study developed a prototype terahertz noise source using photonic methods. By utilizing two beams of incoherent light for photo-mixing in a high-speed photodetector, the system generates terahertz noise with a frequency range of 140~220 GHz, a maximum ENR of 47 dB, and a flatness better than ±2.0 dB. The ENR can also be tuned by adjusting the optical power. Stability tests show that the prototype maintains an ENR stability of 0.35 dB over 12 hours of continuous operation and an output repeatability of 0.39 dB over 10 power cycles. Additionally, the noise figure of a mixer module was measured using the noise source, with the measurement uncertainty being less than 0.48 dB. The development of this photonic terahertz noise source has increased the maximum operating frequency of noise sources in China, providing essential testing instruments for the design and optimization of terahertz devices.