Chen, J. Cao, S.S. Lai, L.W. Leng, Y.B. Yuan, R.X. Zhang, N. JACoW Geneva, Switzerland 978-3-95450-192-2 10.18429/JACoW-IBIC2017-WEPCF01 English 411-414 WEPCF01 ion cavity experiment dipole SRF Contribution to a conference proceedings 2018 2018-03 https://doi.org/10.18429/JACoW-IBIC2017-WEPCF01 http://jacow.org/ibic2017/papers/wepcf01.pdf The measurement of beam charge is a fundamental re-quirement to all particle accelerators facility. In this paper, using the TM010 mode of the cavity BPM to measure the beam charge will be introduced. The data processing methods including harmonic analysis, time domain analysis and principle component analysis (PCA) are used and compared in evaluating the resolution of the beam charge. On the basis of this, the results of the evaluation at a ultra-low charge are also given and indicates the superiority of the cavity probe in the measurement of lower beam charge. In addition, the use of double-cavity mixing method to measure the beam charge will be proposed as well.