The analog signal communicates the information of electronic system to human in real world, especially for the sense of vision and audition. Among specifications of analog signal, Total-Harmonic-Distortion (THD) is the most significant dynamic parameter for audio and video applications. In this paper, we propose a methodology to estimate THD in time domain. Instead of using Fast-Fourier-Transform (FFT), we express the signal under test into Taylor expansion and derive the relationship between coefficients of Taylor series and composition of harmonic. Furthermore, through analyzing syndromes on signal under test with respect to coefficients of Taylor series and extracting effective information, we calculate these coefficients and translate into harmonic distortion. The required instrument of proposed method is oscilloscope which is more conventional than spectrum analyzer. Especially for those low speed voice or physiology analog signals, the required spectrum analyzer is particular and expensive. In addition, the stability of THD estimation by proposed method is better than FFT which its accuracy is proportional to quantity of samples.