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Understanding structure-fluorescence correlation is very helpful for the design of fluorescent probes. In this paper, a donor-acceptor-donor (D-A-D) type NIR-II fluorophore with benzobisthiadiazole as the acceptor and triphenyl amine as the donor, and its three derivatives bearing respectively amino, tert-butyloxycarbonyl amino and phenylazo groups in donor moieties, are synthesized. Their electronic structures and optical properties are investigated via theoretical and experimental studies. It is found that all the three types of substituents significantly influence its fluorescent properties and the phenylazo groups dramatically enhance its quantum yield (QY). To achieve biological applications and maintain high QY in aqueous environments, the phenylazo-containing fluorophore is encapsulated in polystyrene-co-poly(ethylene glycol) micelles. The obtained fluorescent micelles have a QY of ≈3.51% in 1000-1500 nm in aqueous medium that is among the highest of the organic NIR-II probes reported so far for biological imaging. The high QY enables the in vivo imaging of the micelle-administered mice to be conducted with high speed and quality. As an application example, ultrafast NIR-II imaging of intravenously injected mice is performed and used to determine their cardiac cycle and heart rate. The micelles also significantly accumulate in tumors after tail-vein injection and exhibit great application potentials in tumor detection.