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This paper presents an original investigation into the axial evolution of streamer stem during a dark period in long spark discharge. To obtain thermodynamic morphology and temperature distribution of stems, we set up a quantitative schlieren system with the temporal and spatial resolutions of 0.37 μs and 31 μm/pixel, respectively. The quantitative schlieren observation experiments of positive leader discharge with a 1.0 m rod-plate gap were carried out, and the time-resolved quantitative schlieren images were captured. Furthermore, the temperature distribution of stems and its morphology evolution in the axial direction during a dark period were obtained. Due to the dispersion of first streamer discharge, the gas temperature in stem roots shows two evolutionary trends, namely, rising and falling. It was found that the gas temperature in stem decreased along the axis with the increase in the distance from stem root, and the gas temperature of a thermal thin channel was between 400 and 800 K. There is a significant dependency between axial development parameters of thermal thin channels and the first streamer discharge parameters. The phenomenon of channel abrupt elongation triggered by secondary streamer discharge was observed by the schlieren system, and the influence of characteristic parameters on the inception of secondary streamer was statistically analyzed. The ion current waveform in leader relaxation phase was measured, and it is clarified that the generation mechanism of thermal thin channels is due to the energy transfer between positive ions and neutral particles, which finally leads to the increase in gas temperature in the channels. [ABSTRACT FROM AUTHOR]