The study on the reform of the registration system and pricing efficiency is of great significance for protecting investors’ rights, improving market efficiency, promoting enterprise development, and enhancing the pricing efficiency of the capital market. It has attracted extensive attention from academic and practical circles. Using samples from the GEM (Growth Enterprise Market) and the main board companies from 2019 to 2021, this study constructs a DID (difference-in-differences) model to examine the impact of the registration system reform policy on IPO underpricing. The study found that the registration system reform in the GEM has significantly increased IPO underpricing. Mechanism tests show that this phenomenon is particularly pronounced in samples involving less innovative inquiry letters, higher exclusion rates of overpriced subscriptions, and higher investor divergence, providing new evidence for the signal theory in the context of China’s registration system reform. Based on this conclusion, relevant suggestions are proposed to improve the efficiency of IPO pricing.