Recommended as a medical emergency, infectious keratitis with an acute and rapid disease progression can lead to serious damage of vision and even blindness. Herein, we present a kind of theranostic agents, which are made of vancomycin (Van)-modified fluorescent silicon nanoparticles (SiNPs-Van), enabling rapid and non-invasive diagnosis and treatment of Gram-positive bacteria-induced keratitis in a simultaneous manner. Typically, the resultant SiNPs-Van nanoagents have an ability of imaging bacteria in a short time both in vitro (5 min) and in vivo (10 min), making them an efficacious diagnostic agent for the detection of bacterial keratitis. In addition, the SiNPs-Van feature distinct antimicrobial activity, with superior activity of 92.5% at a concentration of 0.5 μg/mL against Staphylococcus aureus (S. aureus); comparatively, the antimicrobial rate of free vancomycin is 23.3% at the same concentration. We further explore the SiNPs-Van agents as eye drops for therapy of S. aureus-induced bacterial keratitis on rat model. Represented by slit-lamp scores, the keratitis severity of SiNPs-Van-treated corneas is 3.4, which is 59.6% and 77.3% slighter than vancomycin-(8.2 score) and PBS-treated corneas (15.0 score), respectively. The infected corneas recover to normal (1 score) after 7-d of SiNPs-Van treatment. Above results suggest that the SiNPs-Van could serve as a new kind of high-quality nanotheranostic agents, especially suitable for simultaneous diagnosis and therapy of Gram-positive bacteria keratitis.