Quality by design (QbD) is applied to the upstream process to maximize the RBD-Fc fusion protein production in CHO cells. The three factors (culture duration, temperature, and polyethyleneimine to plasmid DNA (PEI-Max/pDNA) ratio) were identified as critical process attributes based on risk analysis (FMEA) and further optimized by response surface to maximize the protein yields. Using a Box-Behnken design, the optimal conditions for RBD-Fc production were determined to be a culture duration of 5 days, a culture temperature of 34.4 °C, and a PEI-Max/pDNA ratio of 4.2:1 (w/w) with a predictive value of 48 mg/L (desirability of 92.8 %). The PEI-Max/pDNA ratio and its interaction with culture duration to express the highest yield (47.78 ± 2.30 mg/l). In addition, the purified CHO-produced RBD-Fc fusion protein was highly pure and strongly bound to its receptor, ACE2. Our finding demonstrated that the QBD tools can identify the critical parameters to facilitate scaling-up production.