A single-mode Ar+ laser at 515nm is modulated in amplitude and frequency by an aoousto-optic modulator respectively, the heterodyne saturation absorption signals in I2 are investigated experimentally. A first derivative-like saturation absorption signal with very steep slope obtained from the FM method is used as a discriminator to stabilize Ar+ laser frequency to hyperflne component a3 of the -P(13) 43-0 of 127I2. With the beat frequency between the stabilized Ar+ laser and other stabilized Ar+ laser stabilized to the same line, the stability of the Ar+ laser is better than ±4.7×10-12.