Optical imaging and therapy have attracted intense attention due to their high spatial and temporal resolution and mini-invasive and selective features compared to traditional diagnosis and therapy techniques. Conjugated polymer nanoparticles (CPNs) with excellent light-harvesting performance, high biocompatibility, and tunable molecular structure have emerged as good candidates for fluorescence (FL) imaging, photoacoustic (PA) imaging, and photothermal therapy (PTT) for cancer. Recent studies have demonstrated that CPNs with absorption or emission in the second near-infrared biological window (1000-1700 nm, NIR-II) can achieve high fidelity imaging and efficient treatment of deep tumors because the NIR-II window can reduce the absorption and scattering of photons in tissues, minimize the autofluorescence background, increase the penetration depth, and maximize permissible exposure. In this mini-review, we summarize the currently reported NIR-II CPNs and discuss their applications in NIR-II imaging and cancer PTT. Finally, a brief conclusion is given coupled with perspectives for current challenges and opportunities in this field.