The development of special organic materials that can absorb light energy in both UV-Vis and near-infrared (NIR) regions is significantly important for their appealing applications in highly efficient solar energy utilization. Herein, through introducing a vertically π-extended strong acceptor unit, the absorption maximum peaks of all three donor–acceptor polymers were pushed over 1000 nm. Polymer E-DTP showed an ultralow bandgap of 0.52 eV with an absorption tail extending beyond 2000 nm. Polymer E-T exhibited broad absorption covering the wavelength range from 300 to 1700 nm, which is basically consistent with the solar spectrum. Both E-DTP and E-T exhibit high photothermal conversion efficiencies under 1064 nm irradiation, 58.2% for the former and 50.3% for the latter, suggesting efficient NIR light utilization. Significantly, under one sunlight irradiation, high photothermal conversion with excellent photostability was also achieved for E-T. The excellent photothermal conversion ability of E-T is mainly attributed to the ultrafast internal conversion process (1.49 ps). Through simply dip-coating oil-soluble E-T on the top of a hydrophilic melamine sponge (MFS) as photothermal materials, a Janus evaporator could be easily constructed. Such evaporator exhibited a high water evaporation rate of 2.10 kg m⁻² h⁻¹ with a remarkable solar-to-vapor efficiency of 86.9% under 1 sun irradiation, showing the purification ability of multiple waste waters (seawater, microorganism and organic dyes contaminated waters). This study provides insight into the photothermal mechanism for ultralow bandgap organic materials and a guideline for the rational design of high-performance organic photothermal materials. Moreover, this work indicates the enormous prospects for the NIR polymer toward high performance solar thermal conversion.