Parameter sensitivity indices of power system stabilizer（PSS） are introduced and sensitivity computation function are implemented in SSAP power system software package by two approaches;（i）the sensitivity computation function of eigenvalues associated with oscillation modes and their damping ratios via PSS controller parameters; and（ii）the sensitivity computation function of the eigenvalues of oscillation modes and their damping ratios via PSS parameters subject to the condition of constant equivalent AC gain. Low frequency oscillation damping is optimized in both approaches. The 600 MW generator in Honghaiwan power station is taken for case study. The parameter of PSS is optimized by sensitivity analysis subject to constant equivalent AC gain. The optimized results well coincide with the oscillograms recorded during the on site experiment. The sensitivity analysis by SSAP is validated not only by theoretic analysis but also by engineering practice.