In order to provide a relationship between nanoparticle (NP) parameters and irradiated lights when designing solar heating devices, shape-dependent solar thermal conversion properties of Au nanofluids at different irradiated lights were investigated experimentally and theoretically. Firstly, quasi-sphere and thorny Au NPs were successfully synthesized by a seed-mediated method, and the peak absorbance wavelength of which was located in ranges of 879-553 nm and 899-594 nm, respectively. Experimental results showed that the solar thermal conversion efficiency of Au nanofluids decreased with increasing NP size, which decreased from 86.0% to 58.6% when the NP size increased from 8.5 nm to 138.9 nm. Theoretical results indicated that for these large NPs, the scattering effect plays a major role in the solar thermal conversion process, and using the extinction coefficient to calculate the conversion efficiency may cause a substantial error. Furtherly it can be found that the solar thermal conversion efficiency of thorny Au nanofluids can achieve even 10% more than that of the quasi-sphere Au nanofluids. In addition, the temperature difference between the nanofluids and base fluid water under different incident light using a filter was found to be smaller than that of full solar spectra irradiation. The maximum efficiency of Au nanofluids was similar to 61.7%, the enhancement of which reached similar to 37.1% compared with water at an irradiation light with the wavelength of 500 nm.