The efficient and accurate burn-up measurement of the spherical fuel element is the key component of the operation of the pebble bed high temperature gas-cooled reactor. The accuracy of the method that determine burnup by the activity of Cs-137 degrades due to operation characteristics of HTR-10. HTR-10, as an test reactor, operated on and off during the past years. It stayed shutdown more than power operation. In order to improve the measurement accuracy of Cs-137 activity and enhance the possibility to detect radionuclides with low activity, which can be used to correct the classic burnup assay method, a new measurement system is now discussed using anti-coincidence technology, which suppresses the Compton plateau. In this paper, Geant4 is used to simulate the anticoincidence measurement process taking high purity germanium γ-ray spectrometer as main detector and plastic scintillator as the annular detector. By analyzing the signal to noise ratio in different detection scenarios with all kinds of shape parameters of the annular detector, the annular detector with the best anti-coincidence effect are optimaized. The above research results provide an important theoretical basis for the construction of online burn-up measurement system based on anti-Compton technology.