Objective To compare the effect of applying low molecular weight heparin calcium in different periods of reteplase for acute myocardial infarction(AMI).Methods Ninety-one cases of AMI were divided into group A(30 cases),group B(30 cases)and group C(31 cases)by random digits table.The patients in three groups were given reteplase intravenous thrombolysis,and 5000 U low molecular weight heparin calcium was abdominal wall subcutaneous injections once 12 h at intravenous thrombolysis immediately(group A),after intravenous thrombolysis 6 h(group B)and 12 h(group C).They all were treated for 7-10 d.The rate of coronary artery recanalization,acute phase complication and adverse reaction were compared among three groups.Results The rate of coronary artery recanalization in group A,B and C was 76.7％(23/30),96.7％(29/30),74.2％(23/31)respectively.The rate of coronary artery recanalization in group B was significantly higher than that in group A and C(P ＜ 0.05).The rate of acute phase complication and adverse reaction in group B[33.3％(10/30)]was significantly lower than that in group A and C[70.0％ (21/30)and 61.3％(19/31)](P ＜ 0.05).Conclusions After reteplase intravenous thrombolysis in 6 h,applying low molecular weight heparin calcium can significantly improve the rate of coronary artery recanalization and reduce the rate of acute phase complication and adverse reaction.