Direct absorption solar collector (DASC) has been a prevalent research topic in utilizing the solar energy. Usually, a nanofluid is employed in a DASC so that the solar radiation can be absorbed directly and the heat loss is reduced substantially compared to a traditional surface-based solar collector. Theoretical and experimental studies have been conducted to enhance the absorption performance of nanofluids from the perspective of the nanoparticle's shape and material. Optimization of the DASC has also been performed, mostly from the perspective of the collector's geometry. However, one of the most important parameters, i.e., spectral distribution of the absorption coefficient of a nanofluid, has not been received much attention so far. In this research, the spectral absorption coefficient of a plasmonic nanofluid is optimized for a DASC, so that the thermal efficiency can be maximized while maintaining the magnitude of the average absorption coefficient at a certain value. This will lower the cost of DASC because the absorption coefficient is proportional to the particle concentration and the noble metal, such as gold, silver and copper, is usually used for plasmonic nanofluids. Sufficiently low concentration of nanoparticles can also avoid particle agglomeration which is critical for maintaining stable solar collector performance. We show that when the constraint on the average absorption coefficient is above the saturation level, a uniform spectral absorption coefficient just at the saturation level maximizes the DASC efficiency. On the other hand, if this constraint is below the saturation level, the spectral absorption coefficient following the solar spectrum maximizes the DASC efficiency. In addition, the effect of collector channel height on the optimal distribution of the spectral absorption coefficient is also investigated.