The dependencies of hydrocarbon product distributions of alkali promoted iron catalyst in Fischer-Tropsch synthesis have been studied.The concept of two superimposed Anderson-Schulz-Flory distributions has been applied for the representation of the effects of Mg,La and Ca promoters on product distributions.The FTS performance of the catalysts was tested in a fixed bed reactor under the conditions 563 K,1.7 MPa,H_2/CO=1 and space velocities 4.86 and 13.28 nl·h~(-1)·g_(Fe)~(-1).The results indicate that appropriate amounts of these promoters enhance the break in ASF distributions in the order Ca > Mg > La because of the rising of the catalyst surface basicity.