A test device for focal-plane energy flux density was designed based on a dual-axis tracking trough concentrator system. The focusing loss caused by the positioning error and tracking error of the receiver was studied through theoretical analysis and experimental testing. Furthermore, the optical loss was quantified by the acquisition factor, and laws of the influence of various errors were revealed. The results show that with the increase in the positioning error, the focal-plane width increases, whereas the focal-plane energy flux density decreases and tends to be uniform. Moreover, the focal-plane center migration and optical loss increase with the increase in the tracking error angle. For the dual-axis tracking trough concentrator system used in this experiment, when the receiver aperture is 50 mm, if the acquisition factor is greater than 90%, the positioning error of receiver is required to be between ±1.1% of 455 mm and tracking error angle is required to be less than 0.111°, and the acquisition factor can reach 95%. The acquisition factor is more sensitive to the variation of the tracking error angle. The experimental results are consistent with the theoretical analysis results, verifying the reliability of the test equipment and methods. The function relation of experiment fitting can effectively guide the engineering application.