For the precise measurement, a femtosecond mode-locked Yb-doped fiber laser with a 6 MHz repetition rate tuning range at a fundamental repetition rate of 26 MHz was reported, corresponding to 23% tuning ratio. With chirped fiber Bragg grating, the mode-locked fiber laser could run at different dispersion regimes. The influence of the net cavity dispersion on output characteristics and repetition rate tunability was studied. In a range of negative cavity dispersion, the mode-locked fiber laser could output the nearly same spectrum when the repetition rate was tuned, and the output spectrum was nearly Gaussian-type. Based on this result, a simple scheme, spatial optical path structure, and optimized cavity parameters was designed, which promised the large tunable range and stable mode-locking. The mode-locked fiber could stably output 3.23 mW ultrashort laser pulses with 347 fs dechirped pulse duration. Furthermore, the Yb-fiber laser was locked to the Rb atomic clock so that the repetition rate could be stabilized. The Allan deviation is 2×10^-10 when the average time was 1 s.