We present an approach to calculate the biexciton ground state including the electron-longitudinal-optical-phonon coupling and an accurate variational function to describe the Coulomb correlations in the biexciton. We apply this method to the long-standing problem of biexciton binding energy in CuCl and obtain a binding energy of 32.6 meV. We also discuss the effect of electron-hole (e-h) exchange interaction on the biexciton binding energy. Including correction due to e-h exchange, the theoretical binding energy is 28.8 meV, in good agreement with the experimental value of 32 meV. Details of the biexciton wavefunction are presented in the form of correlation functions with respect to two-particle separations, which show how the particles in the biexciton are distributed.