This paper analyzes the mechanism of Cr4+:YAG passive mode-locking as a saturable absorber of 1064 nm laser. A suitable folding-cavity was adopted to achieve the mode-locking and the intracavity frequency doubling of a Nd:YAG/KTP/Cr4+:YAG laser pumped by a flash-lamp. The picosecond green pulse train with output energy of 12 mJ was obtained.