We experimentally compare the detailed dynamic noise characteristics of a narrow linewidth-swept laser using different tuning mechanisms from three aspects, including the tunable frequency function in the time domain, frequency fluctuation power spectral density in the frequency domain, and its evolution law with time. All the analyses are conducted based on 120° phase difference interferometry and phase real-time reconstruction. The experimental results denote that different frequency-sweeping mechanisms and control parameters exhibit different real-time dynamic noise characteristics. Electro-optic modulation can obtain optimal sweep linearity using an optical phase-locked loop; however, this will result in noise deterioration such as feedback resonance peaks. Simultaneously, the different control parameters applied to the same electro-optic modulator will result in different feedback resonance peak frequencies. Acousto-optic modulation exhibits minimal noise degradation, and piezoelectric modulation results in poor sweep linearity and large tuning noise. This study lays the foundation for the research of the laser-frequency-sweeping mechanism and control technology and the selection of a light source in engineering applications.