This paper describes high − quality welding of magnesium alioy ． At first welding defects were restra 血 ed by pulse wave pa ［ tern cotttrol ． Nex い he defec【s were restrai 員 ed by irradiating simultaneously 芝wo beams ． ］ e pulse wave YAG laser beam and continuous wave YAG laser beam were ¢ ombined to irradiate the magnes 童 um alk ｝ y ． One factor of porosity and crack forma 巨ons was 重 he remain of metal p 鑑 asma inside the molten zone by steep collapse of a keyhole ． Therefore ， the weld 三 ng defec1s occurred easily only in the pulse YAG laser we 聖 ding． Then ， the keyhole was formed by the pu 翼 sc wave YAG laser beam ， and lhe CW laser was combined to prevent rapid cooling ． As a result ， the cracks and the porosities stmcture were completely restrai 蹴 ed ， and Ihe melt diameter and the penetration depth increased， Key Wordsr Magnesium alloy ， YAG 正 aser weld 量 ng ． Porosity ， Crack