The factors influencing Mg segregation layer thickness and ways controlling Mg segregation layer thickness were described by analyzing origination of Mg segregation in 5182 Al-Mg alloy.Mg segregation layer thickness can be controlled in 1.5² mm by adopting LHC mould with double-water-exhausted pore to enhance the cooling rate and cooling water volume at 98 m3/h at initial stage and increasing linearly 239 m3/h at working stage to ensure the match of cooling rate with drawing velocity as well as controlling height of liquid metal at 40mm.