We proposed an optical sensing scheme, based on the mechanism of weak measurement, to detect the sonic pressure transversally without any disturbance. According to the theory of weak measurement in frequency domain, a tiny phase difference between the two eigenstates of the system could be detected, depending on the shift of central wavelength of output spectra. The optical path difference between two paths of a Mach–Zehnder interferometer is induced by the varied acoustic waves propagating in liquid. The propagating direction of the testing light beam is orthogonal to that of the acoustic wave. Thus, the phase shift corresponding to the changing acoustic pressure could be monitored transversally. The experiment with the hydrophone is implemented as a reference to demonstrate the feasibility and authenticity of this weak measurement scheme. With this method, an optical phase resolution of was acquired in such a weak measurement system.