The invention discloses a simplified method and system based on PWM (Pulse-Width Modulation) for a direct-drive fan. The simplified method comprises the following steps: recognizing constituent partsin an original model of a direct-drive fan system; and forming a simplified system model through replacing and simulating the machine side converter in the original model by a machine side controlledvoltage source and a machine side controlled current source, replacing and simulating the network side converter in the original model by a network side controlled voltage source and a network side controlled current source, and adding a direct current unloading protection circuit for the direct current bus in the original mode. The method provided by the invention simulates the machine side converter and the network side converter by using the machine side controlled source and the network side controlled source to greatly simplify the fan system and improve the system simulation verificationefficiency, maintains the direct bus in the system model to effectively reduce the possible instantaneous states such as overvoltage and over-current in the actual operating process and guarantee theeffectiveness of the system model, and provides the direct current unloading protection circuit to improve the reliability and stability of the system model.