The invention discloses a measurement circuit and measurement method for noise signal power and belongs to the technical field of electronic test. The measurement circuit comprises a band switch, a radio frequency processing circuit, a microwave processing circuit, an intermediate frequency switch, a variable gain intermediate frequency amplifier and an analog-to-digital converter. According to the measurement circuit and the measurement method, on the basis of radio frequency and microwave noise power detection voltages and an intermediate frequency pre-detection power value, gain of a variable gain radio frequency low noise amplifier circuit, a variable gain microwave low noise amplifier circuit and a variable gain intermediate frequency amplifier circuit are reasonably set, a whole receiving circuit is guaranteed to be in an optimum linear receiving range, measurement precision of noise signal power is improved, manual intervention is avoided, and precise measurement of various different types of tested piece noise coefficients can be realized.