To investigate the significance of detrusor muscle thickness (DMT) to bladder wall thickness (BWT) ratio as a detrusor-sarcopenia and a consistently applicable factor for noninvasive diagnosis of detrusor underactivity (DU). We prospectively performed a urodynamic study of 100 male with medical refractory lower-urinary-tract-symptoms during 2017–2019. The DMT, BWT and DMT/BWT ratio were measured by ultrasonography every 50 mL during bladder filling, and were analyzed for non-invasive diagnosis of DU and prediction of prostate surgery outcome with questionnaire and the maximum-flow-rate. Of the 94 patients, DU was urodynamically diagnosed in 24 (25.5%). The DMT/BWT ratio was maintained in all patients until the 50% of the maximum cystometric capacity (MCC), and then rapidly decreased. At 20% of the MCC, the DMT/BWT ratio was significantly lower in the DU group (44.0 ± 4.9% vs. 49.4 ± 6.7%, p = 0.008). The DMT/BWT ratio of less than 47.5% at 20% of the MCC showed the ideal accuracy for diagnosing DU (AUC = 0.763), and was a predictor of failure at 12 months after prostate surgery (OR 8.78, p = 0.024). A DMT/BWT ratio of less than 47.5% at 20% of the MCC is a consistently applicable factor for non-invasive diagnosis of DU and could also be considered detrusor-sarcopenia.