The injector upgrade of LINAC II at DESY aims to improve its reliability and mitigate the radiological activation of components due to electron loss at relatively high energy of hundreds of MeV. Therefore, a 2.998 GHz hybrid buncher has been developed and will be installed in between an existing 2.998 GHz pre-buncher and LINAC II. It comprises a 1-cell standing-wave (SW) section for rapid electron acceleration and a 13-cells traveling-wave (TW) section for further beam bunching and acceleration. This paper focuses on its radio-frequency tuning procedure. The tuning strategy combines a non-resonant bead-pull measurement of complex electric field and a linear model for local reflection coefficient calculation. It is demonstrated that imaginary part of the local reflection coefficient represents the field distribution straightforwardly, based on which the structure can be tuned from cell to cell. During tuning, special attention has been paid to the field enhancement in the SW section to ensure its beam-capturing capability. Field amplitude and phase, global and local reflection coefficients have been analyzed for two different frequencies simultaneously, i.e. the intrinsic frequency of the structure and the target frequency, to avoid over-tuning. The tuning result is satisfying. For the target frequency, field unflatness of the TW section has been reduced from plus or minus 9% to plus or minus 4%, and field in the SW section has been enhanced significantly. Meanwhile, in the TW section, the deviation of phase advances between adjacent cells from the nominal value 120 degree has been reduced from the range plus or minus 5 degree to plus or minus 2 degree . By using ASTRA simulation, it has been verified that the residual detuning of the structure is acceptable in view of the beam dynamics performance.