The third-order simplified spherical harmonics approximation to radiative transfer equation is utilized as forward model for the fluorescence diffuse optical tomography (FDOT). This algorithm overcomes the drawback of big computation consumption of PN approximation and limitation that DA is unapplicable to the low-scattering tissue. The time-domain technique offers the potential advantages in directly measuring lifetime and has the best performances of simultaneously recovering of fluorescent yield and lifetime distributions, as well as resolving multiple components. On these bases, an image reconstruction method of generalized pulse spectrum technique based on the SP3 equation(SP3-GPST) is presented, and it is an extent of generalized pulse spectrum technique based on the diffusion equation(DA-GPST). Simulation results show that SP3-GPST performs better than DA-GPST.