Abstract: AZ61 magnesium alloy was welded by gas(argon) tungsten-arc welding (GTAW) process at different heat inputs, the effects of heat input on the average grain size and second phase in weld seam zone of the welded joint were analysed. The results show that the second phase β-Mg17Al12 increased with the incresse of heat input. When the welding heat input increased from 26.0 J·mm-1 to 45.5 J·mm-1, the average grains size in weld seam zone increased from 10 μm to 17 μm, and the area percentage of the second phase β-Mg17Al12 increased from 11.2 % to 13.9 %.