A novel short cavity Er-Yb codoped single-longitudinal-mode frequency modulated fiber laser is demonstrated. The theoretical spectrum expression of frequency modulated laser using delayed self homodyne technique is deduced and simulated. This laser is mainly fabri-cated by a 4 cm long Er-Yb codoped fiber and two spectrally narrow fiber Bragg gratings. To realize the purpose of frequency modulation,the grating near the input of pump is modulated in length. The output power of the laser is more than 3 mW,linewidth is less than 400 kHz and maximum value of the laser optical frequency deviation can reach 55 MHz.