Polyhexamethyleneguanidine phosphate (PHMG) is a prevalent antimicrobial agent. However, its role in humidifier disinfectant-associated lung injuries (HDLI) is poorly defined. This study aimed to clarify the effects of PHMG on the innate immune system, particularly its impacts on the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway and its relation to pulmonary fibrosis. We observed that exposure to PHMG resulted in decreased expression of interferon-β (IFN-β) and interferon responses through a cGAS/STING-dependent mechanism. Furthermore, intratracheal administration of PHMG (1 or 2 mg/kg) in mice induced lung fibrosis, as demonstrated by H&E staining with Szapiel scoring and Masson’s trichrome staining with Ashcroft scoring. Lower doses of PHMG notably increased IFN-β production in the lungs, but higher doses diminished IFN-β levels, suggesting a biphasic impact that initially stimulates inflammation but ultimately compromises host defensive mechanisms, resulting in pulmonary fibrosis. The results emphasize the essential function of the cGAS/STING pathway in PHMG-induced pulmonary damage and indicate that modulating this pathway could effectively alleviate the detrimental impacts of PHMG.