Respiratory detection plays a significant role in the assessment of human health status and in aiding the diagnosis of diseases. This requires the development of fast, economical, and noninvasive detection technologies. This research employed tunable diode laser absorption spectroscopy (TDLAS) to analyze exhaled methane (CH4) levels in patients with gastrointestinal diseases. The system achieved a minimum detection limit (MDL) of 42.12 ppbv, and an accuracy of 8.04 ppbv. A total of 451 samples of exhaled CH4 concentration from gastrointestinal surgery patients were detected, and several gastrointestinal diseases with high average CH4 concentration were screened out. Support vector machine (SVM) algorithms were used to investigate the relationship between CH4 levels and specific gastrointestinal diseases. The study achieved a classification accuracy, recall, and precision rate of 100% for ileus, ascending colon cancer (COAD), and cancers with liver metastasis. Meantime, the rate of CH4 positivity for patients undergoing postoperative chemotherapy was observed to be significantly lower than for patients with active cancer. However, the presence of CH4 was not significantly associated with gastrointestinal stromal tumors of the stomach (GIST), stomach cancer (SC), or rectal cancer (READ), indicating the need for a multifaceted diagnostic approach incorporating additional gas markers. In conclusion, CH4 analysis holds promise as a novel noninvasive diagnostic tool for gastrointestinal diseases, with significant potential for clinical application.