The optimum parameters for laser propulsion are discussed, such as laser induced pressure on targets, interaction parameters (C_(m), I_(sp)) and optimum laser intensity I_(s), etc. It is verified that the larger laser power density will induce higher thrusting force. It is also found that, to drive a 1.010-kg target during confined laser ablation in vacuum and a 17.45-g one during direct laser ablation in air at the standard pressure, the needed minimum power intensities are on the same order of magnitude.