Parabolic trough concentration (PTC) systems are widespread technologies for the large-scale exploitation of solar energy. The flux density on the wall of the absorber of a PTC is non-uniform and complicated, and the calculation of flux distribution is key for estimating the optic performance of a PTC and analyzing the flow characteristic of the fluids in the tubular absorber. Traditional 3D ray-tracing methods for the calculation of flux distribution consume many computational resources and involve long CPU running time. This paper presents a 2D method for the fast calculation of the flux distribution of a PTC based on a descending dimension algorithm. By converting the PTC optical model from 3D space to 2D space, the calculation consumption required by the presented 2D method shrinks by two orders of magnitude compared to traditional 3D ray-tracing methods. Upon assessment, the method demonstrates a capacity to work out the flux density distribution within 0.22 s, compared to approximately 40 s required by traditional 3D ray-tracing methods, using a standard personal computer with a level of good accuracy, with a standard deviation of approximately 0.3 suns. Mathematical proof of this method was also provided. The numerical results were compared with those from the literature and a good agreement was observed, with the average standard deviation of 0.359 suns under different incident angles, proving the reliability of the method presented here. Based on this 2D method, a software tool was developed to facilitate the analysis of characteristics of solar PTC systems.