Parabolic trough solar receivers are the key heat-collecting elements (HCEs) in parabolic trough collectors (PTCs). However, the HCEs emit considerable heat loss at high operating temperature, which exerts significantly negative impact on the thermal performance of the HCE and PTC. For accurately calculating the thermal radiation in HCEs, a novel spectral thermal radiation model based on the spectral parameters was established and validated. Comparison between the simulation results and experimental data demonstrated the model yielded satisfactory consistency. Integrating the achieved spectral thermal radiation with circumferentially maldistributed solar irradiance, a rarely mentioned phenomenon of negative net heat gain and negative energy-flow region (NER) partially around the HCE were discovered. NER exercises adverse influences on the thermal performance of the HCE. In this study, quantitative analyses on the NER by the proposed negative energy-flow index (NEI) were investigated. The effects of parameters, namely, absorber temperature, solar irradiance, on the NER and HCE were studied as well. Besides, a novel optimization strategy by introducing a radiation shield for effectively reducing the heat loss was proposed. The performance of the proposed novel HCE with an inner radiation shield (NHCE-RS) was also investigated. Results showed NEI can scientifically analyze the negative net heat gain and thermal performance in NER, and NHCE-RS has superior thermal performance at higher absorber temperature and lower solar irradiance. The heat loss of the NHCE-RS was effectively reduced by 35.9% at the absorber temperature of 600 degrees C and solar irradiance of 500 W/m(2).