Red and blue light regulate plant development and metabolism. Paris polyphylla var. yunnanensis, for which there is great demand, is a medicinal plant mainly distributed in China and other parts of Asia. However, the effects of red and blue light on the growth and metabolites of P. polyphylla var. yunnanensis remain unknown. This study evaluated the effects of red (R), blue (B), and red-blue composite light (1 R:1B, 2 R:1B, 3 R:1B) on plant growth and accumulation of steroidal saponins (polyphyllin I, II, VI, and VII) in P. polyphylla var. yunnanensis. The results showed that red and 2 R:1B light significantly increased the net photosynthetic rate and the fresh weight of P. polyphylla var. yunnanensis. Also the combined light, especially 3 R:1B, alleviated the damage to membrane lipid peroxidation in plants. Metabolomic analysis showed that different light treatments, especially 2 R:1B, significantly upregulated the levels of metabolites in metabolic pathways related to carbon and nitrogen balance, energy metabolism, and secondary metabolism in P. polyphylla var. yunnanensis leaves. Furthermore, 1 R:1B and 2 R:1B light treatments promoted the accumulation of steroidal saponins, such as polyphyllin I and VI in rhizomes of P. polyphylla var. yunnanensis. Based on the above results, the red-blue composite light (2 R:1B) could be considered as a suitable light treatment to increase the biomass and steroidal saponin contents of P. polyphylla var. yunnanensis.