Deep ultraviolet (DUV) femtosecond laser, which combines the advantages of high single-photon energy of DUV laser with high peak power of femtosecond laser, is widely used in scientific research, biomedicine, material processing and so on. However, in the process of generating DUV femtosecond laser based on nonlinear frequency conversion is encountered a problem that the group velocity mismatch caused by dispersion makes the temporal walk-off of the nonlinear frequency conversion larger than the pulse duration of the femtosecond laser, thus making the generation of the DUV femtosecond laser very difficult. In this work, based on a Yb-doped fiber femtosecond laser, the delay line is optimized to precisely compensate for the spatial and temporal walk-off, so DUV femtosecond laser possesses the following performances: the center wavelength is 206 nm, the repetition rate is 1 MHz, the maximum output power is 102 mW, the maximum conversion efficiency is 4.25% from near infrared to DUV, the root mean square (RMS) power stability is 0.88% within 3 h, and the peak-to-peak power stability is 3.75%. The evolution of laser spectra and beam quality in the process of second harmonic generation (SHG), fourth harmonic generation (FHG) and sum-frequency generation (SFG) are also systematically studied. The experimental results provide a basis for generating DUV femtosecond laser from femtosecond fiber lasers.