We have designed a square-shaped beam homogenizer using a lens array for pumping a petawatt-class Ti:sapphire amplifier. The designed beam homogenizer generated a flat-top uniform square pump beam with 5.1% of edge steepness and 2.7% of RMS spatial uniformity, respectively. This pump beam generated an output beam with 74 J of energy and 56.8% of extraction efficiency, respectively. A scalar diffraction and two-dimensional amplification simulation were performed for optimization. The results indicate the possibility of improving an amplification efficiency and developing a compact square petawatt-class laser system.