In this paper, we report a nonlinear optical parametric amplifier featuring high conversion efficiency, high temporal contrast and ultra-broadband spectrum simultaneously, based on the dual-crystal scheme. By removing the idler pulse between two crystals and optimizing the phase-matching geometry, the effective conversion efficiency can be enhanced to ∼ 40%, and the spectral bandwidth of the amplified pulse can reach 210 nm. Moreover, the degradation of temporal contrast due to the backconversion in dual-crystal scheme is avoided by optimizing two crystals' thickness. The measured contrast ratio is 10 - 10 , which is almost the same as that of the single-crystal scheme. The energy stability and the beam profile in both dual crystal and single-crystal scheme are also compared. This work can provide very meaningful guideline for the design of high-peak-power OPCPA. [ABSTRACT FROM AUTHOR]