Backgrounds: Percutaneous transseptal transcatheter mitral valve-in-valve implantation (TMViV) has become an alternative minimally invasive treatment choice for patients with degenerated mitral bioprosthesis and high surgical risk. However, transseptal approach is more technically challenging than transapical approach in TMViV procedures. Objective: The objective of this study was to introduce the experience of applying long pre-curved sheaths in transseptal TMViV procedures and to evaluate the effect of long pre-curved sheath techniques in TMViV procedures. Methods: Between January 2020 and December 2021, 27 patients with degenerated bioprosthetic mitral valve underwent TMViV procedures using a balloon-expandable valve via the transseptal approach. The regular 14/16F expandable sheath were used for low-profile delivery in first 10 cases, and 22F long pre-curved sheath were used in the next 17 cases during procedures. We retrospectively reviewed the catheter techniques, perioperative characteristics, and prognosis. The median follow-up time was 12 (1–21) months. To further scrutinize our data, we divided the group into the early 10 patients using 14/16F expandable sheath and the subsequent 17 patients with long pre-curved sheath in order to assess the impact of different sheaths and procedural details on outcomes. Results: Procedural success was obtained in all patients with no in-hospital mortality. Seventeen patients received 26 mm prostheses; the remaining ten patients received 29 mm prostheses. Post balloon dilatation was performed in one case. Total procedure time was (96.1 \pm 28.2) min, the fluoroscopic time was (27.4 \pm 6.5) min, and total contrast volume was (50.7 \pm 10.1) mL. One patient received blood transfusion because of hemorrhage at the femoral puncture site. One patient received a permanent pacemaker implantation due to high-degree atrioventricular block at postoperative day 3. There were no other major post-procedure complications and the median length of hospital stay was 4 days. Twenty-five (92.6%) patients improved by \ge1 New York Heart Association (NYHA) functional class at 30 days. In subsequent sub analysis, there were shorter procedural time [(85.2 \pm 24.3) vs. (115.2 \pm 25.6) min, p = 0.0048] and shorter fluoroscopic time [(24.3 \pm 5.2) vs. (31.3 \pm 5.1) min, p = 0.0073] in cases with the long pre-curved sheath than ones with regular expandable sheath. The iatrogenic atrial septal defect (ASD) closure was performed because of the transeptal large right to left shunt in 2 cases with regular expandable sheath, but no patient needed intraoperative ASD closure in cases with the long pre-curved sheath. Conclusions: Transseptal TMViV using long pre-curved sheath could simplify transseptal approach with reliable outcomes for patients of degenerated mitral bioprosthesis.