We demonstrate a novel concept of wavelength-spacing tuning of a dual-wavelength active-mode locking fiber laser in the 1.3-μm wavelength region. A dual-cavity configuration is implemented using a chirped fiber Bragg grating pair for wavelength-spacing tuning. Both normal and anomalous dispersion cavities were schematically designed for wavelength-spacing tuning ranging from 3.52-33.54 nm. The side-mode suppression ratio was ∼30 dB for both wavelengths. Because of the filter-less tuning mechanism, a fast repletion rate of wavelength-spacing sweeping and switching was demonstrated for frequencies up to 50 kHz.