A Si3N4-based antimicrobial coating was fabricated from suspensions with long-time stability. The stability of Si3N4 suspension and the antimicrobial performance of coating were investigated. The viscosity of Si3N4 suspension was found to be the key factor affecting both stability and antimicrobial performance. The Si3N4 suspensions were modified with HASE acrylic copolymer, PAA-NH4, and Polyurethane, resulting in an improvement of the stability of Si3N4 suspension from a few hours to six months. The coating produced using the modified Si3N4 suspension exhibited an antibacterial activity of 99.9%. This study highlights the crucial role of modifying the Si3N4 suspension in improving its long-term stability and coating antimicrobial activity.