High gain greater than 10⁶ is crucial for the preamplifiers of joule-class high energy lasers. In this work, we present a specially designed compact amplifier using 0.5%Nd,5%Gd:SrF₂ and 0.5%Nd,5%Y:SrF₂ crystals. The irregular crystal shape enhances the gain length of the laser beam and helps suppress parasitic oscillations. The amplified spontaneous emission (ASE) induced by the high gain is analyzed through ray tracing. The balance between gain and ASE is estimated via numerical simulation. The gain spectral characteristics of the two-stage two-pass amplifier are examined, demonstrating the advantages of using different crystals, with bandwidths up to 8 nm and gains over 10⁶. Additionally, the temperature and stress distributions in the Nd,Gd:SrF₂ crystal are simulated. This work is expected to contribute to the development of high peak power (≥ Terawatt-class) high energy (Joule-class) laser devices.