High current and intense electromagnetic radiation in the lightning return stroke channel core are the leading causes of many lightning disasters. With the rapid development of modern science and technology, lightning protection is becoming more and more important. In order to perfect the lightning protection system, it is necessary to investigate the microscopic physical mechanism of the lightning channel formation and development from the characteristic parameters describing the channel core. Up to now, spectral observation is the most effective means to obtain the lightning channel core characteristic parameters. In a field experiment on the Qinghai Plateau in China in the summer of 2015, a slit-less grating spectrometer assembled by a high-speed camera as a recording system was used, combined with a fast antenna ground electric field measuring instrument, the spectra of a cloud-to-ground discharge process including four return strokes and the synchronous fast electric field change information were recorded. According to the spectrum and plasma theory, the conductivity of the lightning return channel core is calculated. On this basis, the lightning electrodynamics model is used to calculate the lightning return stroke velocity, peak current, electromagnetic field across the return stroke channel core and peak power per unit length of the channel core. The results show that the return stroke velocity is in the range of (1.2~2.3)×10⁸ m·s^-1, and the maximum values of the axial electric field, radial electric field and magnetic induction intensity across the return stroke channel core are in the range of (1.42~1.74)×10⁵ V·m^-1, (8.22~9.99)×10⁸ V·m^-1 and 1.51~2.83 T, respectively. When the peak current is in the range of 7.52~24.05 kA, the peak power of the return stroke channel core is in the range of (0.63~1.92)×10⁹ W·m^-1. In addition, the correlations between the electrical conductivity, the initial electric field peak value, the return stroke velocity and the peak current and the peak power are analyzed, it is found that the peak current and peak power has a good linear relationship. The results can provide a reference for further exploring the microscopic physical mechanism of the formation and development of the lightning return stroke channel.