Objective: To find out a better criterion to identify septic shock patients after flexible ureteroscopic lithotripsy (FUL). Materials and Methods: In total, 2364 patients who underwent the FUL procedure were enrolled in the study. The demographics and preoperative results of laboratory tests of the patients were collected. The postoperative white blood cell (WBC), systemic inflammatory response syndrome (SIRS), and quick sequential (sepsis-related) organ failure assessment score (qSOFA) were assessed 2 hours after FUL. The predictive efficacy was measured by sensitivity, specificity, positive and negative predictive value, and area under the receiver's operating characteristic curve (AUROC). Results: A total of 15 (0.63%) patients developed septic shock. There were 86 (3.64%) patients who were SIRS positive and 69 (2.92%) patients who were qSOFA positive. The pre- and postoperative WBC ratios in septic shock patients and normal patients were 2.50 ± 1.55 and 0.69 ± 0.24, respectively ( p < 0.001). For sensitivity and negative predictive value, all reached 100%. For specificity, qSOFA was 97.70%, SIRS was 96.98%, and SIRS combining pre- and postoperative WBC ratio (the new criterion) was 99.79%. The new criterion had statistically significant higher specificity than SIRS or qSOFA ( p < 0.001 for both), but when comparing SIRS and qSOFA, it had statistically insignificant specificity ( p = 0.142). For positive predictive value, qSOFA was 21.73%, SIRS was 17.44%, and the new criterion was 75%. qSOFA and SIRS had similar AUROC (0.989 for qSOFA and 0.985 for SIRS), both lower than the new criterion (AUROC: 0.999). Conclusions: SIRS combined with pre- and postoperative WBC ratio has a much better specificity and positive predictive value than SIRS or qSOFA alone. It has 99.79% specificity and 75% positive predictive value, and as high as 100% sensitivity and negative predictive value.