Based on the gold wire bonding process and thin film process, a miniaturized millimeter-wave tunable microstrip filter with low insertion loss is realized in this paper. Firstly, a half-wavelength open circuit tunable resonator loaded with rectangular slots is proposed. The equivalent inductance of the resonator could be changed by bonding gold wires over different rectangular slots, so as to change the resonant frequency. Then, a Ka-band fifth-order tunable bandpass filter is realized by using the proposed tunable resonator, and the frequency is tuned by changing the state of bonded gold wires. Thereby the deviation between simulation and measured results could be corrected, which solves the problems of frequency shift of narrow-band filter caused by the factors of the differences in dielectric constant between batches and the machining errors. In order to verify the feasibility, a fifth-order tunable microstrip filter is simulated, optimized, fabricated and measured. The results show that the passband of the filter can be adjusted from 35.35 GHz to 35.95 GHz with insertion of loss less than 4.6 dB, and the required passband can be obtained.