The underwater streamer discharge has received extensive attention in the field of environmental protection, because it can generate free radicals and reactive oxygen species directly in water. The multi-needle electrode is a basic electrode configuration for achieving large-volume underwater streamer discharge. Understanding the discharge characteristics of the multi-needle electrode configuration is important for designing the large-volume discharge reactors. In this work, a multi-needle electrode that can assemble 21 needles is employed. The number of anode needles generating a streamer discharge during a single pulsed discharge and the differences in morphological characteristics between the inside and the edge of the electrode array are investigated by using an ultra-high-speed camera system. The electric field distribution of the multi-needle electrode is simulated by using the COMSOL software, and the effect of the electric field distribution on the discharge of multi-needle electrode is also studied. The discharge energy efficiency of the multi-needle electrode configuration is evaluated. It is found that the 21 needles are not discharged simultaneously during a discharge pulse. The number of discharged anode needles gradually increases and then reaches a maximum value (≤21). The maximum number of discharged anode needles during a single discharge pulse increases as the voltage and needle spacing increases. During a single discharge pulse, the filament generated from the needles at the edge of the electrode array grows longer and deviates more largely from the needle axis than that generated from the needles inside the electrode array. Such characteristics are primarily due to the disturbance of the electric field among the 21 needles. As the needle spacing decreases, the disturbance of the electric field among the 21 needles gets stronger, consequently, the discharge morphology differences between the needles at the edge and needles at the inner of the needle array become more significant, and the energy efficiency of the discharge drops remarkably.