Objective To evaluate the effectiveness of a modified dynamic needle-tip positioning technique under ultrasound guidance for insertion of mini-midline catheters in adults.Methods A total of 96 hospitalized patients were enrolled and randomized by random number method into three groups (n=32 each). The control group underwent static ultrasound-guided puncture. Experimental group 1 underwent conventional dynamic ultrasound-guided puncture. Experimental group 2 underwent a modified dynamic needle-tip positioning ultrasound-guided technique. Results First-attempt catheterization success rates were 56.25% in the control group, 65.63% in experimental group 1, and 90.63% in experimental group 2; the between-group differences were statistically significant (P<0.05). Catheterization time in experimental group 2 was significantly shorter than in the other two groups (P<0.05). No significant differences were observed among the three groups for guidewire insertion success rate or catheter indwelling time (all P>0.05). Comparison of catheter-related complications after insertion (excluding subcutaneous ecchymosis) showed no statistically significant differences among groups (all P>0.05). Conclusion The modified dynamic needle-tip positioning ultrasound-guided technique is better suited for insertion of mini-midline catheters in adults. It significantly improves first-attempt catheterization success and shortens procedure time, and it reduces the incidence of post-insertion subcutaneous ecchymosis.