The effects of laser welding parameter on strength of welded joints were studied. The mechanism for loss of joint strength was analyzed. It was pointed out that an important factor affecting joint strength is the reaction between matrix and reinforced phase. On the basis of this, the concept of critical St activity a[si]min was proposed. Using appropriate welding parameters and Si activity, high quality laser welded joints in an aluminum matrix composite SiCw/6061Al can be Successfully obtained. The effects of laser welding parameter on strength of welded joints were studied. The was for pointed out that an important factor affecting joint strength was the reaction between matrix and reinforced phase. On the basis of this, the concept of critical St activity a [si] min was proposed. Using appropriate welding parameters and Si activity, high quality laser welded joints in an aluminum matrix composite SiCw / 6061Al can be Successfully obtained.