Objective To analyze 25(OH)D levels and influencing factors in type 2 diabetic patients before and after COVID-19 infection. Methods A total of 1994 inpatients with T2DM were randomly selected from December 2015 to September 2017(before COVID-19 pandemic) and from January 2023 to December 2023(after COVID-19 pandemic). These inpatients were divided into the 25(OH)D level-deficient group, the 25(OH)D level-insufficient group, and the 25(OH)D level-sufficient group. Based on that, we retrospectively analyzed the differences in gender, age, fasting insulin(FINS), fasting plasma glucose(FPG), homa insulin-resistance(HOMA-IR), total cholesterol(TC), high-density lipoprotein cholesterol(HDL-C), low-density lipoprotein cholesterol(LDL-C), triglyceride(TG), uric acid(UA), hemoglobin a1c(HbA1c), as well as urine total protein(UTP) in T2DM inpatients with different vitamin D levels before and after infection with the novel coronavirus. Results The results in patients before infection with the novel coronavirus showed that the levels of FINS, HOMA-IR, TG, HbA1c, and UTP were negatively correlated with 25(OH)D levels(P<0.05), but age was positively correlated with 25(OH)D levels(P<0.05). In addition, the multivariate ordered logistic regression analysis indicated that changes of 25(OH)D levels in patients with T2DM could affect the levels of TG, HbA1c, and UTP, where low levels TG and UTP were considered as protective factors(OR=0.991, P<0.001; OR=0.899, P<0.001), but high levels HbA1c were considered a risk factor(OR=1.002, P<0.002). On the other hand, the results in patients after infection with the novel coronavirus showed that the gender, FINS, FPG, HOMA-IR, and UTP were negatively correlated with 25(OH)D levels(P< 0.05). However, age was positively correlated with vitamin D levels(P< 0.05). Besides, the results of the multivariate ordered logistic regression analysis model indicated that the differences in 25(OH)D levels among T2DM patients were influenced by gender, age, and UTP levels, where UTP was considered as protective factors(OR=0.999, P<0.001), but age was considered as a risk factor(OR=1.023, P<0.001).Conclusion Before and after the infection, the deficiency of 25(OH)D is negatively correlated with the degree of blood sugar control in T2DM patients. Low levels of 24hUTP are protective factors for 25(OH)D deficiency, However, the 24-hour urinary total protein(UTP) levels in patients infected with the virus are significantly higher compared to those not infected