The influence of grating period and noncollinear geometry on the broadly tunable bandwidth of optical parametric amplification are investigated theoretically and numerically for quasiphasematched crystal. The concept of maximum grating period is proposed to achieve the phase matching and groupvelocity matching simultaneously in a wide range. By employing the maximum grating period, geometry Ⅰ is recommended due to the much wider signal tunable range compared with geometry Ⅱ. An expression is proposed to calculate the maximum grating period for congruent periodically poled LiNbO3 with 800 nm pump wave. A feasible scheme is presented to determine the working temperature and noncollinear angle α, maximize the tunable range and simplify the tuning by rotating angle θ only. Finally, the parametric bandwidth is studied with consideration of the influences of the maximum grating period and noncollinear geometry.