The direct absorption solar collector (DASC) is a cost-efficient approach to absorb the solar energy. Many factors would affect the performance of DASCs, including solar radiation, volume fraction, mass flow rate and optical thickness. Among them, the optical properties of nanofluids has decisive effect on the performance of DASCs. We thought up a creative idea for using nonspherical nanoparticles with the influence of mass flow rate. A 2D-Monte Carlo method coupling with discrete dipole approximation (DDA) was proposed to investigate the effect of length-radius ratio, effective radius and incident light angle of Au nanorods (AuNRs) on the optical characteristics and analyse solar weighted absorption coefficient of DASCs. Results indicated that flow characteristics and morphological changes are suggested to signally improve the absorption characteristics of Au nanorod-fluids. The transmittance of Au/water nanorod-fluids (WB-AuNRFs) under specific flow guidance was 19.8% less than that with a disordered state at full spectrum. With appropriate flow regulation, the solarweighted absorption coefficient can reach 61.1%, while that of disordered state was only 52.1%. Meanwhile, a linear relationship between transmittance and the length-radius ratio, as well as the effective radius, was observed by theoretical study.