As a widely-adopted agent for photothermal therapy (PTT), gold nanorods (Au NRs) remain problematic due to the cytotoxicity derived from cetyltrimethylammonium bromide (CTAB) and the comparatively weak and narrow near-infrared (NIR) absorption band. To address this problem, in this study, we propose a shape-controllable and spectrum-adjustable method of synthesis for Au@Ag/Au nanoparticles (NPs) through first coating a Ag nanolayer on the Au NR seed and a subsequent replacement reaction with HAuCl₄ to yield a Ag/Au alloy nanoshell. Results from TEM and UV-vis spectra analysis showed that the thickness of the Ag layers directly determined the shape and size of the NPs, and the formation of Ag/Au nanoshells effectively enhanced the NIR absorbance of the NPs. Remarkably, the optimum Au@Ag/Au nanospheres (NSs) with a diameter of ∼40 nm were revealed to have a broad and intense absorption cross section from 400 to 1100 nm, a ∼4.7 times higher hyperthermic effect than Au NRs, and low dark-cytotoxicity. By using A549 lung cancer as the model, a series of in vitro investigations was performed and demonstrated that Au@Ag/Au NSs could efficaciously kill cancer cells under a 980 nm irradiation. The efficacy of PTT could be further improved by increasing the concentration, incubation time or irradiation time of the NSs. Moreover, a preliminary in vivo study also showed that, after injection into the A549 tumor, Au@Ag/Au NSs could cause an obvious necrosis at the irradiation site. Thus, a novel, promising and highly-effective NIR PTT agent has been developed, which might greatly advance the application of PTT in biomedical research.