At present, the optical losses of crystalline silicon heterojunction solar cells are mostly reduced with textured surfaces. However, this method has a complicated process and poor repeatability and film uniformity; meanwhile, the textured surfaces increase the carrier transfer path and electron-hole recombination, which hinders the improvement of cell performance. In this paper, a double-layer TiO₂/SiN_x antireflection film on planar silicon substrate was designed by OPAL and TFCalc. Considering the solar spectrum distribution and the spectral response of the crystalline silicon heterojunction solar cells, we optimized this antireflection film by taking the film, glass, and substrate as a whole, with the weighted average optical loss as the evaluation function. Furthermore, the designed film was compared with a single-layer ITO antireflection film on textured silicon in terms of the weighted average optical loss. The results show that the weighted average optical loss of the double-layer antireflection film was 4.69%, which was about 1.97 percentage points lower than that of the single-layer ITO film on textured silicon. Also, the absorption loss of the double-layer film significantly dropped. Our findings provide theoretical support to replace textured silicon with planar silicon.