  In order to pursue high-precision beam position measurements for the
free-electron laser (FEL) facilities, cavity beam position monitor (CBPM) is
employed to measure the transverse position which can meet the requirement of
position resolution with a sub-micrometer or even nanometer scale. But for the
pill-box cavity BPM, the possible existed crosstalk between the cavities will
have effects on the accurate measurement of beam position. Two methods, the
principle component analysis (PCA) method and the method of harmonic analysis,
are proposed in this paper to evaluate the crosstalk based on the experiment
dates from the low quality CBPM prototype in Shanghai Deep ultraviolet free
electron laser (SDUV-FEL) facility and high quality CBPM in Dalian Coherent
Light Source (DCLS), respectively. The results demonstrated that these two
methods are feasible in evaluating the crosstalk between the cavities.
