Two silicon drift detectors combine with the new and nonconventional software pulse-height analyzer (SPHA) to measure the time evolution of soft-X-ray spectra, the thermal electron and superthermal electron temperatures. The high-quality soft-X-ray spectral distributions are easily obtained by the new SPHA. Therefore, the measurement accuracies and the time resolutions of thermal electron and superthermal electron temperatures are also improved. The enhancement phenomenon of superthermal electron avalanche during electron cyclotron resonance heating and the pronounced change for the time evolution of soft-X-ray spectra during supersonic molecular beam injection are observed by this system.