Cancer theranostics agents, such as gold nanorods, represent great potential in cancer therapy. However, the big size and the low yield of the gold nanorods reported previously have limited their clinical translation. Therefore, it is significant to develop a new method to prepare the small gold nanorods (width <8 nm) at larger scale. In this report, a modified seedless method was proposed based on the effect of precursor concentration assisted synthesis of high quality small gold nanorods at large scale. The obtained small gold nanorods exhibit high quality and dimension of (18 ± 5 nm) × (5 ± 1 nm). After modified with biological compatibility reagents, the small gold nanorods behave excellent photoacoustic imaging and photo-thermal therapy ability. These results manifest that the obtained small gold nanorods not only realize the improvements of previously limitations, also are thus supposed to pave the way to cancer theranostics in clinic application.