A non-sampling-based Q-factor measuring apparatus and method use a power conversion module to transform the power variation of inputted optical signals in time domain into the variation in other domains, such as optical wavelength, optical polarization and different output ports of optical elements. Taking optical wavelength as an example, different levels of power variation respond different outputs of wavelength variation through the use of a power-to-wavelength conversion module. An optical filter then separates the inputted optical signals with different wavelengths. The power average of a wavelength for its corresponding optical signals is further calculated by a photo detector. Thereby, the information of the power variation for the inputted optical signals at levels 1 and 0 can be obtained, and the Q-factor for the inputted optical signals is easily measured.