The metal heatsink in the Optical Network Terminal (ONT) can couple the electromagnetic interference from high-speed microstrip lines and radiate electromagnetic waves into space. The electromagnetic radiation from the metal heatsink may have a serious impact on the wireless performance of the receiver. Therefore，in the design stage of ONT products，it is necessary to accurately simulate and evaluate the electromagnetic radiation of metal heatsinks.

This paper proposes a simulation method for electromagnetic radiation of metal heatsinks based on field-circuit co-simulation，which can comprehensively evaluate the resonance frequency and electromagnetic radiation intensity of the metal heatsink. Through the collaborative work of circuit simulation and electromagnetic field simulation，this method uses high-speed time-domain signals on microstrip lines as the excitation source of the model，making the simulation model closer to the actual situation. This paper verifies the rationality and accuracy of the simulation results obtained by this method through experiments.

The research results indicate that: Compared with the common simulation method，the electromagnetic radiation intensity of the heatsink obtained by this simulation method is in the order of －70 dBm，which is more in line with the actual level; The size of the metal heatsink is negatively correlated with its resonant frequency; By changing the size of the metal heatsink，the electromagnetic radiation intensity of the heatsink in the simulation is consistent with the measured Total Isotropic Sensitivity (TIS) metrics in the variation trend，and the correlation coefficient is above 0.94.

The proposed method is relatively accurate in simulating the electromagnetic radiation of metal heatsinks. During the design stage of ONT products，this method can be used to estimate the impact of electromagnetic radiation from metal heatsinks on wireless TIS indicators. This allows for targeted measures to be taken to suppress the electromagnetic radiation from metal heatsinks，thereby reducing debugging time during the prototype stage and improving research and development efficiency.