A microstrip tunable band-pass filter based on stub-loaded Stepped Impedance Resonator(SIR) is designed, which is loaded with varactors. The SIR structure facilitates the suppression of higher harmonics and enables miniaturization of the filter. The model proposed in this paper is a multimode resonator consisting of a SIR and two stubs loaded in the center plane of the SIR. First of all, the resonance characteristics of the stub-loaded SIR is analyzed in the even-odd mode analysis method. Then，the independent control of the odd mode and even mode resonant frequency of the filter is realized by loading varactors. The tunability of the center frequency can be realized by adjusting the capacitance of the varactors. The center frequency increases with the increase of the bias voltage of varactors. The tunable filter shows a center frequency tunable range from 0.82 GHz to 1.17 GHz with less than 5 dB insertion loss and more than 10 dB return loss.