Twelve lightning flashes are successfully triggered under the positive atmospheric electric field condition. The discharge properties of the flashes, and the propagation characteristics and mechanism of the involving upward negative leaders are investigated. When lightning flashes are triggered, the average ground atmospheric electric field is around 5 kV/m, with a maximum value exceeding 13 kV/m. Except for one special event showing a discharge polarity reversal (from positive to negative) and producing multiple negative return strokes, none of the remaining 11 triggered lightning flashes involves the subsequent return stroke process. The discharge currents of these flashes are generally of the order of several hundred amperes. The successfully triggered lightning flashes start with the initiation and the upward propagation of negative stepped leaders, of which the average two-dimensional velocity is 1.85 × 10⁵ m/s. For a total of 132 steps captured by the high-speed video camera, the step lengths range from 0.8 m to 8.7 m, with an average of 3.9 m. During the initial stage of the upward negative stepped leader, the current and electromagnetic field present a significant impulsive feature. The mean value of pulse interval, current peak, charge transfer, half-peak-width and current rise time T_10%–90% are 17.9 μs, 81A, 364 μC, 3.1 μs, and 0.9 μs, respectively. The equivalent linear charge density of a single step is 118.5 μC/m. The branching of the leader channel generally takes place together with the stepping process in two ways: the first way is to implement the multiple connections of clustering space stems/space leaders to the leader head within an individual step cycle, and the corresponding current waveform presents a multi-peak structure, with a peak interval of about 2–3 μs (up to 6–7 μs); the second way is to reactivate those previously extinguished space stems/space leaders and to connect them to the lateral surface of the channel.