Cr4+:YAG crystal was used as a saturable absorber in an active-passive mode-locked Nd:YAG laser. The stable and complete mode-locked pulse train was achieved at 1064 nm. The duration of a single pulse from ～0.8 ns to 2.4 ns was obtained by using Cr4+:YAG crystals with the different initial transmissions, and there was a shortest pulse width. The mode-locking dynamic process of Cr4+: YAG crystal was discussed to explain the experimental result, and the effect of excited-state absorption was considered.