Fiber-gratingadditive-pulse mode-locked fiber laser is analyzed in detail. The oscillation conditionsfor the additive-pulse mode-locked laser was found, it was showed that after reachingstable, the laser′sgain, the loss, the coupling efficiency between the two cavities, and the oscillationwavelength will construct an approximate isosceles triangle on the complex plane. Furtheranalysis of the gain of the laser on the starting stage showed that the gain at the maincavity is unsymmetrical with the central wavelength of fiber gratings, which leads theunsymmetry of the spectrum of the output laser pulses. It was also found that in order torealize the laser oscillation the differences between the two cavities′ nominal length should beadjusted at a certain value because of the light penetration into the fiber gratings.