This study examines the effectiveness of content and language integrated learning (CLIL) in primary school English teaching, focusing its impact on the English learning ability and interest of lower-grade students. Two hundred and forty first-grade students were selected from three primary schools in Shenzhen (115 in the experimental group and 125 in the control group) for a one-week intervention. Results showed that students in the experimental group significantly outperformed those in the control group in terms of English learning ability, maintaining a stable advantage across three tests. Additionally, the experimental group’s learning interest increased significantly from pre-test to post-test, whereas the control group remained stable. Moderation effect analysis indicated that students with initially low interest levels in the pre-test experienced the most significant increase in learning interest after the intervention. The research findings demonstrate that CLIL can effectively enhance primary school students’ English learning ability and interest in learning English, with particularly significant transformative effects for students who initially showed low interest. This study provides preliminary empirical evidence for implementing CLIL in China’s primary education context.