To evaluate the efficacy of a nerve stimulator when used with ultrasound-guided double injection in supraclavicular brachial plexus block. We hypothesised that targeting the inferior trunk of the plexus guided by a nerve stimulator would obtain a higher success rate of ulnar nerve blockade than the traditional double-injection technique. A blinded randomised controlled study. Conducted at the University Hospital from October 2015 to January 2016. Ninety patients undergoing upper extremity surgery were randomised into two equal groups. Patients were randomly allocated to a modified double-injection group (MDI group) or a traditional double-injection group (DI group). All patients received 23 ml of a 1 : 1 mixture of 2% lidocaine and 1% ropivacaine during ultrasound-guided supraclavicular brachial plexus block. In the MDI group (n = 45), half the volume was deposited within the brachial plexus sheath guided by ultrasound, next to the inferior trunk and verified by nerve stimulation; the remaining volume was deposited in the main neural cluster. In the double-injection group (n = 45), the first half volume was deposited on ultrasound guidance alone. Sensory-motor blockade of the musculocutaneous, median, radial, ulnar nerves and surgical anaesthesia, performance time, number of needle passes and complications were recorded. The success rate of complete sensory block of the ulnar nerve within 15 min after local anaesthetic injection. Compared with the DI group the MDI group had higher success rates of complete sensory block of the ulnar nerve (93 vs 67%, P = 0.002) and complete anaesthesia (80 vs 56%, P = 0.014) at 15 min, whereas the average performance time was significantly longer (5.08 ± 1.41 vs 4.10 ± 0.64 min, P < 0.001) and the number of needle passes was significantly higher (4.40 ± 1.14 vs 2.87 ± 0.79, P < 0.001). The MDI technique has a higher success rate for complete sensory block of the ulnar nerve within 15 min of local anaesthetic injection. The time needed to perform the block is about 1 min longer than the traditional technique. http://www.chictr.org.cn with the registration number of ChiCTR-IOR-15007588.