Objective To explore the value of the isosceles right triangle principle combined with the triple blood flashback method for ultrasound-guided mini-midline catheter insertion in the upper arm, aiming to provide guidance for clinical practice. Methods A total of 176 elderly patients from the neurology department were randomly assigned to either a control group (n=88), receiving traditional ultrasound-guided mini-midline catheter insertion, or an observation group (n=88), receiving catheter insertion using the isosceles right triangle principle combined with the triple blood flashback method. One-attempt success rate, puncture time, incidence of puncture complications, incidence of complications during duration of catheter use, and catheter dwell time were compared between the two groups. Results Eighty-four patients in each group completed the study. The one-attempt success rate in the observation group was 95.24%, significantly higher than that in the control group (83.33%, P<0.05). The observation group showed significantly shorter puncture time, longer catheter dwell time, lower incidence of difficult catheter advancement and hematoma during puncture, as well as lower incidence of complications (phlebitis, catheter-related thrombosis, infusion difficulty), compared to the control group (all P<0.05). Conclusion The puncture technique using the isosceles right triangle principle combined with the triple blood flashback method can significantly improve the one-attempt success rate of mini-midline catheter insertion, reduce complications during puncture and use of the catheter, and prolong catheter dwell time.