Background: This study aims to evaluate the effects of comprehensive nursing based on the “3H” theory on blood gas parameters, recovery speed and lung function in children with severe pneumonia. Methods: An analysis was conducted on clinical data from 90 children diagnosed with severe pneumonia who were treated at our hospital between November 2022 and December 2023. They were randomly divided into two groups based on the computer random number table method: a study group, which received comprehensive nursing based on the “3H” theory in addition to routine care (n = 45), and a control group, which received only routine care (n = 45). The recovery speed, blood gas indexes, lung function indexes, and family satisfaction were compared between the two groups. Results: Before treatment, there were no significant differences between the two groups in terms of arterial partial pressure of oxygen (PaO2), arterial oxygen saturation (SaO2), arterial partial pressure of carbon dioxide (PaCO2), forced expiratory volume in 1 second (FEV1), forced vital capacity (FVC), maximum voluntary ventilation (MVV), peak expiratory flow (PEF) and the ratio of FEV1 to FVC (FEV1/FVC) (p > 0.05). After treatment, the study group showed higher compliance, family satisfaction, FEV1, FVC, MVV, PEF, PaO2 and SaO2 levels, and shorter times for cough relief, fever reduction, improvement in shortness of breath, disappearance of lung rales, hospitalization and PaCO2 levels. These differences were all statistically significant (p < 0.05). Conclusions: Comprehensive care under the “3H” theory can significantly improve the recovery speed, blood gas levels, pulmonary function and family satisfaction in children with severe pneumonia. The ISRCTN Registration: ISRCTN13393172.