A new method of microwave photonic filter (MPF) with widely tunable passband is proposed and experimentally proved by using an optical-injected Fabry-Perot laser diode. It is based on the basic principle that the FP laser wavelength selectively amplified under optical injection. By applying the optical wave from a tunable-laser source (TLS) to the PM, a phase modulated optical signal is generated, which injects to the slave laser by the circulator. The overall procedure substantially corresponds to a single passband MPF with its central frequency of the passband by changing the injection locking parameters of the TLS. The proposed MPF has been evaluated experimentally. It is achieved that the single passband MPF with a frequency adjustable range of 23 GHz and a bandwidth of 275 MHz. The out of band suppression ratios or insertion loss is 27.9 dB. These are very useful in many places that require wideband tunablity and high frequency.