Pennisetum sinese is a giant grass with a fast-growing and high rooting rate, high sugar, protein content, and high biomass yield, which causes it to be an efficient and economic energy crop of high productivity, application in phytoremediation, and fodder production. The composting system of this grass that is adapted to the simplest formulation is easy and economically feasible in small farms for cultivating oyster mushrooms. In this study, giant grass compost was employed as a substrate for cultivating three Pleurotus species: P. florida, P. pulmonarius, and P. ostreatus to assess their enzyme activities, growth, and yields. Lignin peroxidase (LiP) was the most active enzyme in each species, while other enzymes were differently expressed between species and developmental phases. The average mass of fruiting bodies formed on the giant grass compost was 173.4 g, 166.5 g, and 152.2 g. The biological effectivity was 82.6%, 78.6%, and 72.5% for P. pulmonarius, P. ostreatus, and P. florida, respectively. The obtained results indicate the usefulness of giant grass compost for the cultivation of the three studied Pleurotus species.