Objective. To explore the correlations between vitamin D level and bone mineral density (BMD), insulin resistance (HOMA-IR), and muscle mass in patients with senile type 2 diabetes mellitus (T2DM). Methods. Totally, 80 patients with senile T2DM admitted to China-Japan Friendship Hospital from Jan 2020 to Oct 2021 were enrolled and assigned to the 25 (OH)D-deficiency group (n = 35) or 25(OH)D-normal group (n = 95) according to serum 25(OH)D level. BMD and HOMA-IR in the femur neck and muscle masses of upper and lower limbs were compared between the two groups, and Pearson's correlation analysis was performed to determine the relations between 25(OH)D and BMD, HOMA-IR, and muscle masses of upper and lower limbs. Results. No notable difference was found between the two groups in general data including age, gender, diabetes duration, BMI, HBA 1c, and fasting insulin (all P > 0.05). Compared with the 25(OH)D-normal group, the 25 (OH)D-deficiency group showed a notably lower BMD in the femur neck, notably lower muscle masses of upper and lower limbs, and a notably higher HOMA-IR level (all P < 0.05). The Pearson's correlation analysis revealed positive associations between 25(OH)D and BMD and muscle masses of upper and lower limbs in patients with senile T2DM and a negative correlation between 25(OH)D and HOMA-IR (all P < 0.05). Conclusions. The serum 25(OH)D decreases notably in patients with senile T2DM, and higher serum 25(OH)D level may improve insulin resistance, limb muscle masses, and bone density and thus maintain bone health.