The resistance and the thermal effects of current in lightning discharge channel are important to the lightning disaster research and prevention design, and the thermal characteristics of lightning channel are closely related to the spectra of the plasma. In this work, using the spectra of two cloud-to-ground lightning with multiple return strokes obtained by a slit-less spectrograph, and combining with the synchronous electrical information, the electrical conductivity, the peak values of current, the arc channel radius has been calculated, then the resistance per unit length, the thermal power per unit length at the instant of peak current, and the heat energy per unit length during the first 5 μs in the channel are firstly obtained. Meanwhile, compared to the common metal conductors, the relationships between the thermal power and the resistance and the square of the peak current in lightning plasma channel are analyzed. The results shown that the resistances per unit length are estimated by spectra to be 0.04～8.41 Ω·m-1. The thermal peak powers per unit length are in the range of 0.88×108～2.20×108 W·m-1 and the heat energies over the initial 5 μs are in the range of 1.47×102～3.66×102 J·m-1. The values of these parameters are consistent with the values reported in other works; The thermal power at time of peak current increases linearly with the resistance but reduces exponentially with the square of the peak current; Because the resistance in lightning channel is inverse ratio to 2/3 power of the temperature, the stronger peak currents are generally corresponding to the higher temperatures, but the resistance and the thermal power will decrease rapidly with the temperature, which further proves why the plasma cannot be heated by the ohmic heating method.