In order to obtain 3-D temperature distribution and time constant of a thermoelectric MEMS microwave power sensor, a 3-D equivalent circuit model was established. Firstly, the equivalent circuit model was established based on the equivalent relationship of thermal parameter and electrical parameter with the structural of sensor. Then the unit module of the equivalent circuit was analyzed theoretically. Finally, the temperature distribution and response time of the sensor were studied according to the 3-D equivalent circuit model. The sensitivity of the sensor is 0.076 mV/mW @10 GHz, and the time constant is 56.24 μs from the model. The measured results show that the sensitivity of the sensor is 0.06 mV/mW @10 GHz, and the time constant is 85 μs. The 3-D equivalent circuit model established in this paper can not only obtain the time constant of microwave power sensor, but also accurately obtain the heat dissipation in the substrate. Therefore, this work has a certain reference value for thermoelectric MEMS microwave power sensors.