To interpolate the missing data existing in RFID data streams, a space-time with weight algorithm is presented in this paper. Combining the ideology of sample interpolating method with mean value interpolating method, the algorithm employs the smoothing window technology and endows sample value and mean value with different weights, respectively. The sum of the two values with weight is adopted to interpolate the missing data. Experiments are carried out. Compared to sample interpolating method and mean value interpolating method, the presented algorithm performs well on data processing rate and interpolating accuracy rate.