Keyhole-induced Porosity formation is a severe problem in partial penetration laser welding. Different from the mechanism of traditionally metallurgical pores (e.g. H2, CO pores), the porosity is mainly caused by instability of capillary keyhole and traditional methods employed to prevent metallurgical pores are ineffective in suppressing keyhole-induced porosity. for partial penetration laser welding of 3～5 mm sheet plates the method and mechanism of porosity prevention by pulse modulation was investigated. The experimental results reveal that pulse modulated laser welding can effectively suppress keyhole-induced porosity formation. The variation of porosity ratio as pulse frequency has regularity, that is, the porosity ratio decreases with increasing pulse frequency under 50 Hz, and remains in low level when pulse frequency lies in a wide range of 50～150 Hz. In addition, the optical signal of plasma was detected in pulse modulated laser welding to explain the mechanism of porosity prevention. The results reveal that the porosity ratio varies well with the standard deviation value of plasma signal, which indicates that the mechanism of porosity suppression lies on the fact that the pulse modulated laser welding under adequate frequency can make keyhole stable.