Two methods,including Mg(OH)2 co-precipitation and direction dilution,were compared in the determination of multiple major/trace elements in sea water by inductively coupled plasma mass spectrometric (ICP-MS) method.Suitable analytical methods were defined respectively to provide practical means for the determination of these elements in seawater by ICP-MS.The results showed that,Mg(OH)2 co-precipitation could achieve the separation,enrichment and accurate determination of 19 elements including V,Cr,Mn,Co,Cd and rare elements(La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu)， while 10 fold direct dilution method could be accurately and simultaneously used to determine 13 major/trace elements including B,Sr,Li,Rb,I,V,Cr,As,Cd,U,Mo,Cu,Mn,but it is not suitable for the detection of Zn,Ni,Co,Pb and low concentration elements like rare earth elements.The two methods are suitable for simple and rapid determination of elements with high accuracy and precision.Combining the two methods together,accurate determination of 28 elements,including Co,La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu(15 elements,by Mg(OH)2 co-precipitation) and B,Sr,Li,Rb,I,V,Cr,As,Cd,U,Mo,Cu,Mn(13 elements,by 10 fold direct dilution) could be achieved with about 50mL of offshore and open sea water.