Abstract: It is critical for the safe and reliable operation of high temperature gas-cooled reactor (HTGR) to accurately monitor the running of the fuel balls in HTGR. To overcome the disadvantages of present detectors, a novel external mounted sensor was presented based on the theory of through-transmission eddy current testing. A numerical computation model of the sensor for electromagnetic field analysis was established by using finite element method. The parameters of the structure and exciting circuit were analyzed to optimize the design. Experimental results show that the sensor takes a high signal-to-noise ratio and has an excellent resolution for continuous passages of fuel balls, which meets the requirements of application in HTGR.