This application discloses a tag quantity estimation system and method of RFID. A processor-readable medium was disclosed at the same time. This estimation method applies a spatial diversity gain existing in a multi-antenna system. Separated and sequentially stacked the real parts and the imaginary parts of the multiple signals received by multiple antennas. Then, a tag quantity estimation problem is converted into a data clustering problem in high-dimensional space. In this way, the overlapped cluster data in low-dimensional space can be separated in the high-dimensional space, thereby improving the accuracy of tag quantity estimation.