The invention provides an online repair device and method of a valve rod of a large-sized valve and belongs to the technical field of chemical device repair tools and methods. According to the technical scheme, the two ends of a connection rod are provided with a rotating shaft and a shaft sleeve correspondingly, the rotating shaft is connected with an electro-hydraulic actuator, an expansion sleeve is installed in an inner hole of the shaft sleeve, the outer wall of the expansion sleeve is matched with the inner hole wall of the shaft sleeve, the inner wall of the expansion sleeve is matchedwith the outer wall of the valve rod, the portion, on the lower portion of the expansion sleeve, of the inner wall of the shaft sleeve is provided with a flat key and is connected with the valve rod in a matched manner, a gland is located at the upper end of the expansion sleeve, the gland is fixedly connected with the top of the shaft sleeve through a gland fastening bolt, the outer wall of the shaft sleeve is sleeved with a copper sleeve, and the copper sleeve is installed in a connection rod installation hole of the valve seat. The online repair device and method of the valve rod of the large-sized valve is the first creation on the aspect of online repair of valve rods of large-sized valves, the difficult problems plaguing production are solved, online repair of transmission failure and work with faults can be conducted on the valve rod, production loss and equipment loss which are caused by halted repair are completely avoided, remarkable economic benefits are achieved, potentialhidden danger in production is eliminated, and the repair device and method of the valve rod of the large-sized valve are worthy of popularization in the industry.