Mode-locking fiber laser has many merits such as compactness, stability and good mode. Using a novel transmitted type semiconductor saturable absorber mirror （SESAM） as an output coupler, a Yb³⁺ doped double-clad Q-switching mode-locking fiber laser is constructed . The shortest period of the envelope of the Q-switched mode-locking is about 500 ns with a repetition rate of 110 kHz and an average output power of 45 mW. The repetition rate of the mode-locking pulse train is 26.7 MHz. The setup of mode-locking laser using transmission type SESAMs is simple and easy to adjust, than that using reflection type SESAMs. So it＇s convenient for insert dispersion compensation elements to achieve femtosecond pulse mode-locking laser.