Face-on radiography is a significant method to diagnose Rayleigh-Taylor (RT) instability in inertial confinement fusion (ICF). The effects of convergent geometry on the measurement of the perturbation amplitude of spherical capsule by face-on radiography are discussed in this paper. By calculation, the actual value and measured value of perturbation amplitude are analyzed under a simplified model. The effects of such factors as offset distance, the outer radius of capsule, the wavelength and amplitude of perturbations on the relative errors of the experiment results are discussed as well. The calculation results show that reasonable choice of these parameters can make the diagnosis of systematic error less than 3%, and the measurement results can be corrected by the calculation model. The study results can provide reference for the parameter design and result analysis of future experiments of convergent geometry erosion RT instability on Shenguang-III laser facility.