Solid-state lithium batteries possess numerous advantages, including high energy density, excellent cycle stability, superior mechanical strength, non-flammability, enhanced safety, and extended service life. These characteristics make them highly suitable for applications in aerospace, new energy vehicles, and portable electronic devices. However, lithium dendrite growth at the electrode/electrolyte interface remains a critical challenge, limiting both performance and safety. The growth of lithium dendrites within the electrolyte not only reduces the battery’s Coulombic efficiency but also risks piercing the electrolyte, leading to internal short circuits between the anode and cathode. This study addresses the issue of lithium dendrite growth in solid-state lithium batteries by employing phase-field theory for numerical simulations. A phase-field model is developed, coupling the mechanical stress field, thermal field, and electrochemical field, to investigate the morphology and evolution of lithium dendrites under different ambient temperatures, external pressures, and their combined effects. The results indicate that higher temperatures and greater external pressures significantly suppress lithium dendrite growth, leading to fewer side branches, smoother surfaces, and more uniform electrochemical deposition. Increased external pressure inhibits longitudinal dendrite growth, resulting in a compressed morphology with higher specific surface area and compactness, though at the cost of increased mechanical instability. Similarly, elevated ambient temperatures enhance lithium-ion diffusion and reaction rates, which further suppress dendrite growth rates and sizes. The combined effects of temperature and pressure exhibit a pronounced inhibitory influence on dendrite growth, with stress concentrating at the dendrite roots. This stress distribution promotes lateral growth, facilitating the formation of flatter and denser lithium deposits.