A full-scale phase demodulation approach based on two quadrature power ratios is proposed to perform photonic instantaneous frequency measurement (P-IFM). In the proposed approach, a delay-line interferometer (DLI) and two laser sources are used. The wavelengths of two lasers are located at the peak point of and at the $-3\mathrm{dB}$ point of the comb filtering response of the DLI. Therefore, at the two wavelengths, a cosine-shape and a sine-shape filtering response are formed to process the $+1$ st or $-1$ st order sidebands, which are generated by externally modulating the light waves of the two lasers with a microwave signal. After being filtered, two quadrature power ratios, cosine-shaped and sine-shaped responses, are obtained at the two wavelengths via power detection and power comparison. The phase term induced by the frequency is then demodulated from the two power ratios without ambiguity within the full-scale range of $0–2\pi$ , the so-called full-scale phase demodulation, which has been verified in a proof-of-concept experiment. Unlike a half-scale range of $0–\pi$ , such a full-scale range of the proposed approach makes it a potential solution to design parallel or cascaded configurations to P-IFM to enhance the measurement range and the resolution.