An approximate analytical wave function is presented for an oblate ellipsoidal big N Bose-Einstein condensate(BEC) in a xi-symmetr ic harmonic traps. The relationship between the undetermined parameter C and the total particle number N,the aspect ratio λ of the BEC is obtained by varying t he energy functional. The parameter Cvaries extremely slowly versus N (orλ),and it becomes a fixed value 0.321646 when both Nand λare very large.