Underground gas storage is the main form of peak regulation of natural gas supply and demand, which has been developed rapidly in recent years. In order to ensure the safety and reliability of oil–gas reservoir construction, it is necessary to seal the used wells one by one. In some difficult used wells, the cover layer of the gas storage reservoir was opened, and at the same time, accidents such as falling fish occurred, resulting in inability to enter from the original hole. Magnetic guidance drilling technology is required to drill a new relief well, and subsequent work requires the use of casing window opening technology in the upper part of the cap layer from outside the casing to the inside of the casing for construction operations to ensure that the cap layer is effectively blocked. To address the issue of creating windows in the casing of gas storage facilities, a three-dimensional mechanical model of window cutting for P110 grade casing was established using Johnson–Cook material constitutive relationship. Finite element analysis was performed to study the effect of tool cutting angle and depth on cutting loads. The numerical simulation result shows that tool angle is negatively related with cutting load, the cutting depth is positively related with cutting load. Moreover, the impact of cutting depth on cutting load is greater than tool angle. Through range analysis, the optimal cutting parameters were identified as a tool angle of 60° and a cutting depth of 0.05 mm. Experimental validation confirmed the feasibility of the scheme, the use of outer casing window technology can safely, efficiently and low-cost to realize the inner and outer casing communication, and then re-inject cement to seal, completely solve the risk of gas emergence caused by such difficult used wells, make a batch of storage to be built from impossible to possible, and make an important contribution to ensuring seasonal peak regulation of natural gas and energy security.