The coupled equations of four_wave mixing by femtosecond pulses in gas_filled capillary are derived. Based on the equation s, the in fluence of the walk_off and phase modulation is investigated. It is found that t he discrepancy between the theoretical and experimental results decreases when t he influence of the walk_off and phase modulation is taken into account. At the same time, it is also found that taking the influence of the 3rd harmonic gener ated by the idler light into account is necessary for reducing the discrepancy b etween the theoretical and experimental results.