Abstract: Full waveform inversion is a new seismic imaging method, which mainly uses full waveform information to inverse parameters of medium in the subsurface, and realizes waveform inversion by minimizing the residual between observed wavefield and theoretical wavefield using nonlinear optimization methods. The objective function of wavefield residual was established based on time domain acoustic wave equation. To test the capability of conjugate gradient(CG) algorithm and BFGS Quasi-Newton algorithm, waveform inversion was applied to a hierarchical model. The inversion effect was compared from wavefield precision, objective function convergence and running time respectively. The CG method was used to reconstruct velocity model on positive, reverse fault and Marmousi models. The inversion results show that the CG method has higher computation efficiency and inversion precision. The CG method is a promising method for full waveform inversion.