In order to study the performance of the thermoelectric MEMS microwave power sensor after packaging, a package of COB technology was proposed. Firstly, the microwave characteristics before and after packaging were simulated by finite element simulation software HFSS. Then, the thermoelectric MEMS microwave power sensor was fabricated in a GaAs MMIC technology, and the prepared chip was packaged. Finally, the microwave characteristics and output characteristics of the sensor were measured before and after packaging. The measured results showed that the return loss after packaging was less than -10.50 dB in the frequency range of 8~12 GHz. The sensitivity before packaging was 0.16 mV/mW@10 GHz, and the sensitivity after packaging was 0.18 mV/mW@10 GHz. After the thermoelectric microwave power sensor was packaged, the output voltage still had a good linearity with the input power. Therefore, this work had certain reference value and guiding significance for the research on the packaging of thermoelectric MEMS microwave power sensors.