The invention discloses a composite cross arm stress strain online monitoring system. The composite cross arm stress strain online monitoring system comprises a sensor module, wherein the sensor module is connected with a first electro-optical conversion module and a second photoelectric conversion module, the first electro-optical conversion module is sequentially connected with a first photoelectric conversion module, a signal conditioning circuit, an A/D conversion module and a digital signal processor; the digital signal processor is connected with an alarm module and a power supply module; the digital signal processor is connected with an upper computer; the power supply module is connected with a second electro-optical conversion module and the second photoelectric conversion module in sequence. The invention further discloses a monitoring method for the monitoring system, wherein the monitoring method comprises the following steps of arranging a strain plate of the sensor module on a node of a composite cross arm, converting the collected strain signal into an electric signal; after processing the electric signal to obtain an electric signal suitable for an AD sampling range and converting the electric signal into a digital signal, inputting the digital signal into the digital signal processor and the upper computer. The monitoring system and the monitoring method thereof can be used for realizing real-time dynamic monitoring of a state of the composite cross arm structure.