Objective To evaluate the clinical efficacy of vitamin D supplementation in patients with postherpetic neuralgia (PHN) and vitamin D deficiency, and to explore the impact of serum vitamin D levels on their prognosis. Methods A randomized controlled trial was conducted in Department of Pain, the Affiliated Hospital of Xuzhou Medical University from August 2023 to December 2023. According to the random number table, 36 patients who met the inclusion and exclusion criteria were divided into two groups (n=18): an experimental group (group A) and a control group (group B). Both groups received basic treatment (medication and nerve block therapy). In addition, group A was administered with vitamin D for a period of 12 weeks, while group B did not receive vitamin D supplementation. The Numeric Rating Scale (NRS), Generalized Anxiety Disorder‑7 (GAD‑7), Patient Health Questionnaire‑9 (PHQ‑9), and Pittsburgh Sleep Quality Index (PSQI) scores were compared at admission, and after treatment for 1 week, 4 weeks, 8 weeks, and 12 weeks. The incidence of PHN in both groups was also compared, along with the amount of analgesics used, the number of rescue medication instances, and the occurrence of adverse reactions during hospitalization. Results At treatment for 12 weeks, group A showed lower NRS, GAD‑7, PHQ‑9, and PSQI scores than group B (all P<0.05). The number of cases that progressed from herpes zoster to PHN, average daily usage of acetaminophen‑tramadol, and the rescue medication usage rate were higher in group B than those in group A (all P<0.05). Compared with admission, both groups showed decreases in NRS, GAD‑7, and PHQ‑9 at treatment for 1 week, 4 weeks, 8 weeks, and 12 weeks, as well as decrease in PSQI scores after treatment for 4 weeks, 8 weeks, and 12 weeks (all P<0.05). Compared with treatment for 1 week, group A presented decreased GAD‑7 scores after treatment for 4 weeks, 8 weeks, and 12 weeks, and decreased PHQ‑9 scores after treatment for 12 weeks (all P<0.05), while group B showed reduced GAD‑7 scores after treatment for 8 weeks and 12 weeks, and decreased PHQ‑9 scores after treatment for 12 weeks (all P<0.05). Compared with treatment for 4 weeks, group A showed decreases in NRS scores after treatment for 8 weeks, and reduction in GAD‑7, PHQ‑9, and PSQI scores after treatment for 8 weeks and 12 weeks (all P<0.05); while group B had decreases in GAD‑7 scores after treatment 8 weeks and 12 weeks (all P<0.05). No statistical differences were found in the remaining indicators (all P>0.05). Conclusion Supplementing vitamin D in patients with postherpetic neuralgia and vitamin D deficiency can improve clinical outcomes.