In this paper, a theoretical and numerical analysis of the factors that affect mode_locked pump threshold was made. The key effect of cavity mode in Kerr_lens mode_locking was illustrated. At the same time, a soliton model showed the rela tion between the pump threshold and dispersion in the cavity. Based on the gene ral analysis, a self_starting femtosecond Ti:sapphire laser with mode_locking th reshold as low as 390 mW was demonstrated.