Abstract: Expressions used for extracting the alignment parameter of photofragment from the laser-induced fluorescence（LIF）intensity are derived by employing the density matrix approach. The alignment of photofragment is described by molecular state multipoles. Both lasers used to dissociate parent molecule and to excite photofragment are linearly polarized lights. And detection fluorescence is an unpolarized light . The LIF intensity is a function of the initial molecular state multipoles，the line strength factor and the dissociation-excitation geometrical factor. The alignment parameter of photofragment can be found by measuring the polarization ritio of fluorescence and calculating the dynamic factors.