In order to alter the output frequency of optoelectronic oscillator (OEO),a kind of tunable OEO based on laser external modulation is demonstrated. In this scheme,an electrical amplifying selective-frequency cavity composed of microwave filter,electrical attenuator,and electrical phase shifter and so on is added to the single-loop OEO. It generates tunable output RF signal by adjusting the bias voltage of electrical phase shifter,which equals to changing the length of electrical selective-frequency cavity. In addition,the oscillation mode of OEO can be changed by adjusting the optical delay line (ODL). The tunable OEO can be established by electrically injection locking between the output signal of selective-frequency cavity and the free oscillation signal of OEO,whose output frequency is determined by the selective-frequency cavity locking the corresponding oscillation mode of OEO. The experimental results show that the output signal with the frequency tuning range of 10.05 GHz~10.09 GHz,which is equivalent to 40 MHz,and the tuning step of 417 kHz is obtained. Its side mode suppression ratio is 60 dB when the output frequency is 10.0519 GHz,and the phase noise is -115 dBc/Hz @10 kHz. With a simple structure,this scheme not only preserves the advantage of low phase noise as OEO,but also effectively suppresses the side mode,which provides a new implementation method for tunable OEO.