Multipole components of potential field are calculated by using a boundary element method in an RF ion trap, when there exist assembling imperfections and axial holes in the end-caps and a band-cut on the equatorial surface of the ring electrode. The variation of multipole components versus the dimensions of holes and band-cut are analyzed in detail. The calculated results indicate that when specific conditions are satisfied, some non-linear effects can be compensated partially each other and a nearly ideal quadrupole potential can be obtained.