This paper presents a data processing algorithm for the Retarding Potential Analyzer (RPA) to retrieve real time parameters of single ion for the electric propulsion plume diagnoses. By analyzing characteristics of response curves of RPA under different conditions, a statistical method is established to derive particle temperature and density through the U-I measurements. A numerical experiment is carried out to simulate an onboard observation with specific requirements on the measurement. Results show that the averaged error in derived ion density is 6.62 %, while the averaged error in ion temperature is 7.75 %. The processing time of one curve is 0.5 s by using the 12 MHz 80C31 microprocessor.