In this paper,the application requirement and traditional method of pulse width compression and broadening of picosecond laser are introduced,and the principle of pulse width compression and broadening using phase mismatched frequency doubling crystal in the cavity is discussed.A class I phase matched LBO crystal is inserted into the cavity of a passively mode locked Nd∶YVO4 semiconductor saturable absorber(SESAM) laser pumped by a semiconductor,the pulse width can be compressed from 8.7 ps to 4.8 ps,and the pulse width can be broadened to 12.8 ps,42.8 ps or even 63 ps by inserting KTP crystals with different thickness and phase matching.The compression and broadening of pulse width are discussed in principle,and the application range of SESAM mode locked laser in laser ranging,laser processing and laser radar detection is extended,which has certain reference value in laser ranging,laser processing and lidar detection.