Abstract: By the improved Tessier method,the paper takes the Lou soil as the object to study the change rules of combined forms of exogenous heavy metals Pb,Zn and Cd with different rate depending on time after they are injected into the soil.The result shows that after the exogenous heavy metals Pb and Zn into the Lou soil,the percentage bound to organic matter remains injection of unchanged as 20% and 15% respectively.The time for the balance of the exogenous heavy metals Pb,Zn and Cd prolongs as the injection rate increases if there is no external disturbance.Pb stays mainly as residue after small rate and medium rate are injected,reaching about 30% and 40% respectively.If large rate is injected,the alternation of Fe-Mn oxides bound to carbonate and bound to play the main role.The combined form relation after Zn injection is residual form> bound to Fe-Mn oxides > bound to organic matter > bound to carbonate > exchangeable form.The form of Cd is hard to reach the dynamic balance within short period after it is injected.