This study aims to demonstrate if this intracavitary electrocardiogram guided method is superior to conventional surface prediction length method in terms of single-time target rate of correct tip placement. In addition, we would like to investigate symptomatic thrombosis incidence and its risk factors after three-valve PICC implanted among patients with malignant tumors; to clarify procedure time used and cost of the new intracavitary ECG guiding method and to explore the maximal P-wave amplitude of intracavity ECG real-time positioning technology and its predictive factors.