Abstract: In present work, the widely used Mg alloys ZK61, AZ31, AZ61 and AZ91 were selected for investigating the solidification cracking susceptibility during welding. Transverse motion weldability (TMW) test in two-speed mode was used to evaluate the susceptibility of Mg alloys to solidification cracking during welding. The critical transverse moving speed was obtained and considered as an index to estimate the solidification cracking susceptibility of Mg alloys as ranking: ZK61 > AZ31 > AZ61 > AZ91. The ranking is consistent with the results of one-speed test and predicted results based on │dT/d(f_S)^1/2│_max. It was shown that two-speed test of TMW is effective to evaluate the susceptibility of Mg alloys to solidification cracking. According to the backfilled liquid at the tip of or near the solidification cracking, the intergranular backfilled liquid of ZK61 is discontinuous and the channel is narrow, which is difficult to heal the crack and increases the solidification cracking susceptibility. The liquid backfilling channel of AZ91 is the widest, which is conducive to intergranular liquid backfilling and reduces the solidification crack susceptibility.