It is significant for the design and production of detonators and other initiating explosive devices to study the variation law between high-pressure spark discharge characteristics and loading high pressure and spark gap. Taking the picosecond time-resolved streak camera and spectrometer as the core and the spark plasma generated by the high-voltage pulse power supply as the research object, a set of transient spectrum and scanning pyrometer systems was built. In the experiment, the two influencing factors of the tip gap and the two-pole voltage are considered. By collecting the transient spectrum of the spark generated by the high-voltage breakdown of the air at the tip, after processing and calibrating the data, combined with Plank’s blackbody radiation theory, the diagnosis of the transient spectrum and temperature of the discharge plasma is realized. The results show that under the same tip gap, with the increase of voltage, the emission spectrum of discharge plasma increases and reaches saturation in a specific voltage range. The transient temperature generated by the discharge shows an overall upward trend and usually gets a peak between 1.02-1.16 μs. When the tip voltage is 10 kV, the temperature can reach up to 16 617 K. In the case of the same voltage, as the gap increases, the emission spectrum of the discharge plasma first increases and then decreases, and the discharge temperature gradually drops in the overall trend.