The invention relates to a method for diagnosing an embedded nonfatal fault in a PLC (programmable logic controller). The traditional fault diagnosis has the defects of occupying a large part of a CPU (central processing unit) and influencing the control efficiency of the PLC. According to the method provided by the invention, the task in the PLC is divided into a control task and a diagnosis task, wherein the control task is used for performing a ladder diagram control algorithm program, thereby controlling a machine and writing a machine data volume into a public data area; the diagnosis task is used for reading the machine data volume in the public data area, inputting the acquired machine data volume into a reasoning engine, and then performing a nonfatal fault diagnosis process; and the two tasks are scheduled by using a priority preemptive scheduling algorithm in the system. By using the method provided by the invention, the defect of low control efficiency of the single task PLC in the prior art is overcome. According to the method, the nonfatal fault diagnosis process is singly used as one task and the control task and the diagnosis task are scheduled by using the prioritypreemptive scheduling algorithm, thereby increasing the PLC control efficiency.