A tunable single-passband microwave p hotonic filter is developed based on phase modulation and stimulated Brillouin s cattering.Theoretically,by introducing two pump signals whose frequency in terval equals 2times as large as the stimulated Brillouin frequency shift,the loss spectrum generated by the pump signal with low frequency is offse t by the gain spectrum of the pump signal with high frequency.As a result,a tuning range of 44GHz is achieved which is 2times as large as the range of a single pump signal.With the increas e of the number of pump light,the tuning range of the filter will grow exponentially.Under a si ngle pump and two pumps,we test the filter.The results are presented,which demonstrate a 0.8－18.2GHz and 0.8－31.1GHz measured tuning ranges,respectively,together with a band width of 26MHz and a out-of-band rejection of 26dB.