Accurately acquiring real-time cooling load of internal heat sources is of great importance for developing energy-saving control strategies for heating and air conditioning systems and reducing the operating energy consumption of buildings. The delayed cooling load generated by the radiation heat transfer process poses a challenge in calculating the internal thermal load. In this paper, the radiant cooling load monitoring model of internal heat sources, represented by a 10th-order transfer function, is developed based on the analysis of heat storage-release process, heat transfer characteristics, and transfer function principle. In order to simplify the calculation process and improve the accuracy, radiant time factors are adopted to identify the parameters and reduce the order of developed models, and a transfer function of order 2 is acquired. An experimental system is built to validate the developed model and the results show an average absolute percentage error of 8.19% for the model. The developed model for thermal lag cooling load in buildings can be applied to building energy consumption monitoring platforms to achieve online load monitoring. It has theoretical significance in describing the time-delay patterns and amplitude attenuation characteristics of the delayed cooling loads. This model provides a theoretical basis for making energy-saving control strategy of the heating and air conditioning system.