By using comprehensive data obtained in rocket-triggered lightning experiment, upward negative precursors at the triggering wire tip as during the ascent of the rocket were studied. Except for those typical precursors with peak current of tens of amperes, abundant of weak current pulses were recognized, attributed to the high vertical resolution current detection. All the isolated current pulses were found to be of the weak ones, and all those typical-intensity precursors (or clustered precursors) were preceded by the weak current pulses, involving a time interval of about 20 μs. For the weak pulses, the impulsive pulses and ripple pulses, we obtained their geometric mean values of peak current (3.6 A, 32.2 A, 11.1 A), risetime from 10% peak to 90% peak (0.39 μs, 0.9 μs, 3.2 μs), duration (2.8 μs, 5.1 μs, 12.7 μs) and charge transfer (4.7 μC, 50.8 μC, 83.2 μC). The typical-intensity precursors formed visible discharge channels that can be detected by optical means, and the channel development of those clustered precursors involved stepwise features consistent with the initial sustained upward leaders, with average 2-D speed in the order of 105 m/s. The temporal and spatial relationship of adjacent precursor-producing channels was analyzed. The new precursor channel was found to initiate at the height of the previous channel tip. The significant adjustment of charge distribution due to the stepped channel extension reduced the electric field intensity in the channel region. The precursors were actually un-sustained leader development, which produced initial leader channel segment at the triggering wire tip but eventually extinguished due to the insufficient conditions.