A computer system and a diagnostic method thereof are provided. The computer system comprises a system management bus (SMBus) switch, a plurality of servers and a remote management controller (RMC). Each server comprises a diagnostic message port, a basic input output system (BIOS) and a logic circuit. The BIOS outputs a diagnostic message to the diagnostic message port. The logic circuit catches the diagnostic message. The RMC comprises a SMBus host controller. The SMBus host controller controls the SMBus switch to connect the SMBus host controller to a corresponding logic circuit according to a request. The logic circuit responds the diagnostic message to the SMBus host controller according to the request.