The effect of third-order nonlinearity on the second harmonic generation of ultrashort pulses in KDP crystal is analyzed theoretically.It is found that the influence of third-order nonlinearity on conversion efficiency becomes dominant when the incident intensity is above 100 GW/cm~2.A novel approach in which an initial phase-mismatch is introduced to compensate for third-order nonlinearity effect is proposed.The calculated results show that for a 50 fs pulse and a 250 GW/cm~2 input intensity,the conversion efficiency is increased by a factor of 1.1 at the condition of 0.9 mrad initial phase-mismatching angle.At the same time,the fluence modulation of the fundamental and second harmonic pulses due to third-order nonlinearity is suppressed effectively and the temporal profiles are improved.