The Precipitation Radar (PR) onboard Tropical Rainfall Measuring Mission (TRMM) is the first rain radar in space. Research on PR retrieval algorithms is important in data application as well as algorithm development. This paper first introduces the PR’s standard algorithms of radar reflectivity factor retrieval used by NASA and JAXA, and then presents an improved retrieval method. Equivalent raindrop diameter is computed for each radar resolution cell according to its rain rate. If a cell doesn’t obey Rayleigh scattering condition, the corresponding radar reflectivity factor will be corrected before rain attenuation correction. Qualitative and quantitative comparisons are made on 5 km×5 km×1.5 km grids among ground radar data, PR standard results and the improved results. Results show that the method improves retrieval accuracy at the cost of a small increase in calculation.