By using the multiple-scale method,we analytically study dynamical properties of two-component Bose-Einstein condensates trapped in a harmonic plus quartic anharmonic potential.It is shown that the anharmonic potential has an important effect on the dark solitons of the condensates.In particular,when the strength of the anharmonic exteal potential increases,the fusion of the two solitons becomes faster.This implies that the fusion of the two solitons can be controlled by an anharmonic potential.