To achieve the requirements of high sensitivity, high efficiency, simple operation, and low sample size for the detection of pepsinogen I (PGI) in large-scale early screening of gastric cancer, a thin-film Lamb wave device with PGI antibody functionality was constructed in this study. The thin film in the device detection cavity was modified with self-assembly of the PGI antibody, which specifically captured and immobilized the PGI antigen in the samples on the surface of the thin film. The resultant increase of mass on the surface led to change of the A0 mode center frequency, and the variation was positively correlated with the increment of mass absorbed on the chamber surface. Thus, the PGI antigen concentration in the samples was detected. The results showed that, when measuring the level of the PGI antigen, the thin-film resonance Lamb wave biosensor with PGI antibody functionality had a sensitivity of about 102.114 Hz/lg (ng/mL), theoretical limit of detection (LOD) of 0.176 ng/mL, and detection time for one sample of 40 min. Compared with the existing PGI detection technology based on optical detection, the Lamb wave sensor has the advantages of being a simple detection system with simple operation and no need for professional operators, and lower LOD than most optical detection methods, two orders lower than that of the electrochemical method of PGI detection. In conclusion, the thin-film resonance Lamb wave biosensor with PGI antibody functionality proposed in this paper has the advantages of low LOD, high sensitivity, high detection efficiency, and simple operation, which can satisfy the basic requirements of large-scale early gastric cancer screening.