The maximum depth of pneumatic DTH drilling with simultaneous casing is difficult to determine. Hence, a theoretical calculation method for the maximum depth of drilling with simultaneous casing is proposed based on the conversion of impact energy and strain energy using the energy method. The formation friction value for drilling is improved and supplemented using an analog method based on relevant technical specifications. Finally, case studies are conducted to analyze and validate the results. The results obtained are as follows: (1) A more systematic and comprehensive calculation method for the maximum depth of drilling with simultaneous casing is proposed by innovatively adopting the theory of energy method. (2) A method for calculating the maximum drilling depth with simultaneous casing in a single formation and a process for calculating the maximum drilling depth in multiple formations are obtained. (3) The basis and method for reducing friction between the casing and formation are proposed. (4) The concept of neutralization point is proposed, and the principle of strain energy acting on the casing string under the action of double impactors is obtained. (5) An increase in the impact energy and the casing thickness can slightly increase the maximum drilling depth with simultaneous casing.