The invention relates to a nuclear reactor core measuring system of a reactor of a pressurized water reactor nuclear power plant, specifically relates to a neutron flux measuring thimble tube replacing process of a nuclear reactor core measuring system and a specific device thereof. The method comprises the following steps of: firstly confirming the requirements of replacing irradiated thimble tube and the position of reactor core of the irradiated thimble tube and the accurate length, and then confirming whether the irradiated thimble tube to be replaced is broken; for the thimble tube prevented from braking, the process for taking out the reactor core of the thimble tube is carried out in one time; for the thimble tube broken, the process for taking out the reactor core of the thimble tube is carried out by sectioning; and the method further comprises a step of inserting a new thimble tube of which the size is in line with that of the irradiated thimble tube to be replaced after confirming the thimble tube is normal. The neutron flux measuring thimble tube replacing process of a nuclear reactor core measuring system and the specific device thereof provided by the invention have the advantages that the security of replacement of the thimble tube is ensured, the operation of the tool is reliable, the application is convenient, and the cost is low.