Objective To explore the prediction of postoperative coronal limb alignment by the tibia fibular angle (TFA) and femoral fibular angle (FFA) after osteotomy in medial open-wedge high tibial osteotomy (MOWHTO). Methods A clinical data of 20 patients with medial compartment osteoarthritis, who were treated with MOWHTO between September 2019 and September 2020, was retrospectively analyzed. The angle and height for open-wedge was planning preoperatively by osteotomy master software, and the TFA and FFA were measured by software after simulated osteotomy. The intraoperative angle for open-wedge was adjusted according to TFA and FFA after simulated osteotomy. Among them, there were 9 males and 11 females; the age ranged from 46 to 69 years, with an average of 56.0 years. Body mass index (BMI) was 21.3-35.7 kg/m2, with an average of 26.7 kg/m2. Osteoarthritis involved 11 cases of left knee and 9 cases of right knee; the disease duration was 2-6 years, with an average of 3.8 years. According to the Kellgren-Lawrence classification, there were 7 cases of grade 1, 9 cases of grade 2, and 4 cases of grade 3. The lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), joint line convergence angle (JLCA, mechanical femorotibial angle (mFTA), weight-bearing line (WBL) ratio, TFA, and FFA were measured before operation and at 2 days after operation. The difference (X) between the intraoperative measurement value and the preoperative plan value of TFA/FFA, and the difference (Y) between the postoperative WBL ratio and the 62.5% were canculated, and the correlation of the difference between the two indicators was analyzed by Pearson's test. According to the median BMI of patients (25.81 kg/m2), the patients were allocated into high BMI group (>25.81 kg/m2, n=10) and low BMI group (≤25.81 kg/m2, n=10), and the influencing factors of WBL ratio was analyzed by linear regression. Results There was no significant difference between pre- and post-operation in LDFA and JLCA (P>0.05); while there were significant differences between pre- and post-operation in MPTA, mFTA, and WBL ratios (P<0.05). The measured values of TFA were (89.5±4.0)° during operation and (87.7±4.7)° after operation, showing significant difference (t=2.991, P=0.008). There was a positive correlation between the difference between the intraoperative measurement value and the preoperative plan value of TFA (X) and the difference between the postoperative WBL ratio and the target force line 62.5% (Y) (r=0.544, P=0.020). The measured values of FFA were (86.9±4.3)° during operation and (85.7±4.4)° after operation, showing significant difference (t=1.760, P=0.094). There was a positive correlation between the difference between the intraoperative measurement value and the preoperative plan value of FFA (X) and the difference between the postoperative WBL ratio and the target force line 62.5% (Y) (r=0.481, P=0.043). After BMI stratification, X was an influential factor of Y in the low BMI group (P<0.05), but X was not an independent factor of Y in the high BMI group (P>0.05). Conclusion Intraoperative FFA and TFA by fluoroscopy can predict coronal limb alignment after MOWHTO. TFA can be used for all patients, and FFA for patients with BMI≤25.81 kg/m2.