The primary discharge circuit composed of switch and capacitors is the basic unit of fast linear transformer driver (FLTD). Under the condition that the amplitude and rise time of the output current are constant, the number of the units and the gas switches can be reduced effectively by reducing the inductance of the unit circuit, thus the overall reliability of FLTD can be improved. Therefore, reducing the inductance of the primary discharge circuit is always the key technology of FLTD. A low inductance FLTD primary discharge unit is developed based on switch and capacitor assembly technology in this paper. The unit uses ring-shaped high voltage pulse capacitor as energy storage element. Its operating voltage is 100 kV, capacity is 39.6 nF, and internal inductance is 8.8 nH. The working characteristics of the primary discharge unit are tested. The experimental results show that when the charging voltage of the capacitor is ±50 kV, the short-circuit output current amplitude of the primary discharge unit is 40 kA, the rise time is 50 ns, and the loop inductance is 101 nH. When the charging voltage of the capacitor is ±80 kV, the output current amplitude of the primary discharge unit to the matched load is 30 kA and the rise time is 66 ns.