A Mach-Zehnder interferometer is connected outside the dissipative soliton mode-locked fiber laser to realize the ultrashort pulse output of the dissipative soliton molecule with controllable separation. The Mach-Zehnder interferometer is constructed by two 50∶50 optical coupler and a tunable optical time delay line. The separation of the dissipative soliton molecule can be adjusted with the optical path difference of the Mach-Zehnder interferometer, by adjusting the time delay line. The dissipative soliton molecules with the pulse separation of 0.8 ps, 1.3 ps, 2.37 ps, 4.25 ps, 6.24 ps, 9.4 ps, and 15.3 ps are observed, respectively, the corresponding spectrum separation are 8.3 nm, 6.1 nm, 3.83 nm, 1.88 nm, 1.27 nm, 0.85 nm, and 0.52 nm. The generation mechanism of the soliton molecule are analyzed theoretically, which agree well with the experimental results. The work provides an effective way to realize dissipative soliton molecule with controllable separation.