Effects of Ca impurity on microstructures and mechanical properties of Al-5Mg filler metals were investigated with SEM, EDS, XRD, tensile test and impact test. The results indicate that the phase composition of Al-5Mg alloy is changed by the Ca impurity. It is observed that block-like (Ti,Cr)₂Ca(Al,Mg)₂₀ phase distributed discontinuously along the grain boundaries when the Ca content is less than 0.28%. When the content of Ca is over 0.28%, bar-like Al₂Ca and block-like (Ti,Cr)₂Ca(Al,Mg)₂₀ combine together and distribute along the grain boundaries, and the amount and size of Ca-rich phase (both the bar-like and block-like) increase with the Ca content. Tensile strength of the Al-5Mg alloys increases firstly and then decreases with increasing Ca content, and the tensile strength gets to maximum value when the content of Ca is 0.28 wt%. The plasticity and fracture toughness decrease smoothly (Ca 0-0.28%) but then significantly (Ca>0.28%), and the tensile or impact fracture mode transform from the transgranular ductile fracture to the brittle cleavage fracture.