The invention makes a request of protecting a safety and reliability evaluation method for a flue gas online monitoring system with consideration of a common cause failure. A typical fault mode of a flue gas on-line monitoring system is analyzed; a common cause failure is processed by a display modeling method by combining a fault tree analysis method; fault events are classified into independentfailure events and common cause failure events and a fault tree model considering the common cause failures is established; and an alpha factor model is used for analyzing the common cause failures ofthe system, so that the safety and reliability of the system are evaluated. According to the invention, the reliability evaluation considering the common cause failure is carried out by using a sample processing subsystem in the flue gas online monitoring system as an example; and reliability calculation is carried out on the system considering the common cause failure based on fault maintenancedata, so that a reliability analysis result more conforming to the actual operation condition is obtained.