Frequency-doubling is investigated using two BBO crystals with 70-fs, 800-nm laser pulses at the repetition of 10 Hz . The size of crystals are both 10 mm×10 mm×0.2 mm . Focused on spatial walk-off and phase velocity mismatching, the angle between the axes of the two crystals and the phase velocity mismatching of the fundamental pulse are adjusted respectively. It is experimentally demonstrated that the spatial walk-off could be compensated by use of two crystals placed anti-parallel, and angle mismatching with some degree may compensate phase velocity mismatching of the frequency-doubled pulse. The frequency-doubled pulse with bandwidth of 6.7 nm and energy of more than 2.8 mJ is achieved. The frequency-doubling conversion efficiency reaches 35.7%, which is improved by a factor of 1.7 related to a single crystal in the same condition.