Objective To explore the best in vitro PICC measurement in children with blood cancer. Methods 70 cases of hematology-oncology patients who aged from 1 to 7 years old from June,2009 to July, 2010 in Pediatric Hematology Oncology were selected and divided randomly into the control group and the observation group with 35 cases in each group. The measurement of the control group was from the pre-puncture point along the vein to the right chest acromioclavicular joint, the measurement of the observation group was from the pre-puncture point along the vein to 1cm below the right chest acromioclavicular joint. Results The rate of PICC placed in the lower superior vena cava was 94.29% in the observation group, and 57.14% in the control group, the difference was significant. Conclusions For children patients aged from 1 to 7 years old with blood cancer, PICC catheter length measurement from the prepuncture point along the vein to 1cm below the right chest acromioclavicular joint, can improve accuracy for the PICC into the lower superior vena cava.