Noise Figure (NF) is an important parameter in evaluating the performance of transmitting a signal from a high-frequency electronic device. As the operating frequency increases, the NF of high-frequency electronic devices usually increases, and the Excess Noise Ratio (ENR) of existing noise sources cannot meet the associated measurement requirements. Therefore, to meet the measurement requirements for the NF of high-frequency electronic devices, we propose combining three incoherent optical beams into an unitraveling carrier photodiode (UTC-PD) based on incoherent optical mixing technology. A tunable terahertz (THz) photonics noise source with a high ENR in the 220-325 GHz frequency range is developed. The ENR can be tuned up to 45 dB. By using the Y-factor method, the proposed THz photonics noise source is applied to measure a THz mixer with large NF and negative conversion gain. The measured NF of the THz mixer ranges from 16 to 32 dB, the conversion gain is about -13 dB, and the uncertainty is 0.43 dB. The tunable THz photonics noise source with high ENR can meet the measurement requirements of THz electronic devices with high NF. It will play an important role in the measurement of NF of THz electronic devices and in guiding further optimization.