An on-site test method for thermal and optical performances of parabolic-trough (PT) loop for utility scale concentrating solar power (CSP) plant is proposed. This method is comprised of sequentially off focus and in-focus tests. The off-focus test is first conducted for thermal performance as that in a complete PT loop, the front (upstream) collectors are used to heat up the heat transfer fluid (HTF) flowing through the rear (downstream) off -focus ones (the test section) while being cooled by the ambient. The front-rear order is reversible to test all the collectors. The correlation between the heat loss and the absorber (outer surface)-ambient temperature difference of the collectors is obtained by fourth order polynomial data-fitting. Then, the in-focus test is carried out for optical performance, which is achieved based on the energy balance. The heat loss is calculated based on the correlation acquired in the off -focus test. The incident solar radiation, heat gain, cosine loss and end loss are calculated based on the meteorological data, experimental data, local time and astronomical conditions, respectively. Therefore, the optical performance, i.e. the optical efficiency, is readily achievable. The proposed test method was implemented on the 300 kWt experimental rig located in Langfang, Hebei, China. The optical efficiency is evaluated to be (70.77 +/- 1.08)%, which lies within the published range. On the other hand, a thermohydraulic model for the parabolic-trough collector (PTC) loop of the 300 kWt experimental rig was developed for system-level use. This model was incorporated with the optical efficiency obtained on the experimental rig. The good agreement between the simulation results and experimental data leads to the reciprocal verifications between the proposed on-site test method and the thermohydraulic model. (C) 2017 Elsevier Ltd. All rights reserved.