Objective:To study the effect of heparin sodium(125 U/ml) for locking on the prethrombotic state in lung cancer patients with peripherally inserted central catheter(PICC). Methods:Clinical data of 20 lung cancer patients with PICC were randomly select-ed in our department. Heparin sodium(125 U/ml) was used for locking after PICC being inserted. All parameters in our study includ-ing prothromhin time(PT),activated partial thromboplsatin time(APTT),thrombin time(TT),fibrinoge(Fbg),platelet(PLT) and D-dimer were dynamically monitored 2 times:0~1 d before(T0) and 2~3 d after(T1)PICC insertion. Results:Differences in PLT and D-dimer between T0 and T1 were significant(t=2.387,P=0.028;t=2.487,P=0.022). There was no significant difference in PT,APTT,TT,Fbg,FDP between T1 and T0. Conclusions:①Using heparin sodium(125 U/ml) for locking is safe for patients with lung cancer after PICC insertion. ②Heparin sodium(125 U/ml) for locking can reduce PLT and D-dimer levels of patients with lung cancer after PICC insertion and can prevent venous thrombosis.