A design of four- layer broadband antireflection coating (ARC) for high efficiency GaAs solar cell [inverted metamorphic 3 junction (IMM-3J),4 junction (IMM-4J) lattice-mismatch solar cell] is proposed. The broadband antireflection coating is prepared on the solar cell by electron- beam thermal evaporation method. According to the differences of reflectance and external quantum efficiency (EQE) before and after evaporation, the short circuit current gain of each junction is calculated. Compared with the double-layer antireflection coating, the four-layer antireflection coating with a wide anti-reflective area is more appropriate for IMM-3J and IMM-4J solar cell. Process stability of the antireflection coating preparation is analyzed by Essential Macleod software. The related design optimization is also given out.