To improve the efficiency of traditional engineering methods in route performance simulation calculation and meet the requirements of domestic civil aircraft mobile and airborne performance applications, a method based on theoretical formulas is proposed to calculate key cruise speeds. The error analysis of cruise Mach number was carried out using the ARJ21 aircraft performance software PES, and the theoretical formula was finely revised by discussing the classification of flight weight and pressure altitude. The modified self written program was compared with PES in terms of computational efficiency and accuracy, and the effectiveness of the modified formula was verified by combining actual QAR data. The simulation experiment results demonstrate that the modified analytical formula for Mach number calculation has high accuracy, and this calculation method can effectively improve the calculation efficiency of key cruising Mach numbers.