The methods for increasing the conversion efficiency of the type Ⅰ and type Ⅱ second harmonic generation with KDP crystal for the ultra intense femtosecond laser pulse have been discussed in detail. The results show that group-velocity mismatching is the main reason of the broadening of the second harmonic pulse and could lead to the decrement of conversion efficiency. The effects of nonlinear phase-modulation cause the distortion of the shape of the second harmonic pulse and also decrease the conversion efficiency. For the type Ⅰ second harmonic generation, the proper spatial phase-mismatching can be helpful in compensation for the decrement of the conversion efficiency caused by the self- and cross-phase-modulation. For the type Ⅱ second harmonic generation, the proper time predelay to the incident fundamental extraordinary pulse can efficiently increase the conversion efficiency, and also restrain the broadening of second harmonic pulse.