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Semiconducting polymers have been widely used for phototheranostics due to their strong absorbance in near infrared region, high photostability, excellent photothermal conversion ability. In this paper, a heavy atom free tri-component semiconducting polymer PTVT has been designed and synthesized by a Stille coupling reaction. PTVT has a high singlet oxygen quantum yield (1O2 QY) of 42.2% in DCM remains high 1O2 generation ability for the DSPE-PEG coated PTVT nanoparticles (NPs) obtained by a nanoprecipitation approach. Owing to the PDT and PTT synergistic therapy (photothermal conversion efficiency ~52.6%), PTVT NPs show a low half maximum inhibitory concentration (IC50) of 5.9 μg/mL towards human lung cancer cells (A549) with laser irradiation. And simultaneously the dark toxicity of such NPs is almost negligible even at high concentrations. Furthermore, in vivo study demonstrates that the NPs are able to inhibit tumor proliferation with laser irradiation, compared with the control and dark group, suggesting the low dark toxicity and high phototoxicity of such NPs. The H&E stained pictures of the normal tissues indicate the biosafety of PTVT NPs in that no obvious damage of heart, liver, lung, kidney and spleen was observed. The results provide an approach to designing heavy atom free semiconducting polymers for phototheranostics.