For the calculation of wave-current force on horizontal cylinder a modified Morison's equation is used.A redefined Keulegan-Carpenter number KC2 is determined for the horizontal cylinder in wave-current co-existing field.The force coefficients are well related to the redefined KC2 number.As to the comparison with the force on vertical cylinder, the characteristics of force on horizontal cylinder are quite similar to those on vertical cylinder, but the force coefficients for horizontal cylinder are larger than those for vertical cylinder.It is proved by the authors' calculation that the results of monochromatic wave can be used directly for the determination of irregular wave-current force on horizontal cylinder in time domain.