With the burn-up measurement system for the 10MW high temperature gas-cooled reactor, the burn-up of the spherical fuel element is obtained indirectly by measuring the gamma-ray from the fission product ¹³⁷Cs. The accuracy of results will affect the security and the economy of the reactor. In this paper, on the basis of the existing equipment conditions of HTR-10, the diversion experiment of the elevator is designed and implemented. By making the spherical fuel element gradually deviate the running measurement place, the relative position between the center of the spherical fuel element and the axis of the collimator is changed. and the relationship between the angle of the diversion and the count rate is obtained and is then used to determine the offset in the circumferential direction between the center of the spherical fuel element and the collimator axis. The model of the HTR-10 burn-up measurement system, established by the MCNP program, can be used to simulate the gamma photon transporting process from the spherical fuel element, through the elevator, the sealed flange and the collimator, to the HPGe detector. The MCNP model can be used to simulate the experiment in different radial deviations. By analyzing the experiment results and calculating results of the MCNP model, the radial diversion from the core of the spherical fuel element to the axis of the collimator can be estimated.