In the KEK Linac, about 100 monitors (BPMs) are used for the beam orbit measurement. The previous data acquisition (DAQ) system consists of the VME and the digital oscilloscope. The maximum DAQ rate is about 1 Hz which is limited by an oscilloscope performance. We have developed the new DAQ system since we have an ongoing linac upgrade project aiming a fast beam-mode switch, in which the fast beam position measurement is strongly required up to 50 Hz. The new BPM DAQ system is a WindowsXP-based fast digital oscilloscope. The DAQ software has been initially developed by C++ and TekVISA. It acquires the waveform signal from the BPM electrodes and calculates the beam positions. It is very useful to run the DAQ software as an EPICS IOC since the most part of Linac parameters are accessible via EPICS protocol and they are archived by EPICS archiver. Recently, we developed the BPM DAQ software based on EPICS which can make the oscilloscope work as an EPICS IOC. The results of performance test show that the new DAQ software can work well up to 50 Hz. In this paper, we will describe the fast BPM DAQ software based on EPICS and the results of its performance evaluation in detail.