<p>The boreability index is a crucial parameter reflecting the difficulty of rock excavation during the rock-cutting process of tunnel-boring machines. Its empirical relationship with rock parameters and rock mass classification is widely used. However, traditional boreability indexes, such as field penetration index and torque penetration index, are affected by operational parameters. This study proposed an improved index based on theoretical simplification and field penetration tests to eliminate its influence. First, theoretical relationships between thrust, penetration depth, rock strength, and tunnel design parameters are derived. Based on this, improved indexes are defined as the linear fitting coefficient between the thrust, torque, and penetration depth per revolution from rock-cutting data. Then, field penetration tests with specified and unspecified operational setting values were carried out in the Songhua River and Chaoer to Xiliao River water diversion tunnels. Field results indicate that the improved index can perform better in thrust and torque prediction than traditional field penetration and torque penetration index. Finally, another field penetration test from a double-shield TBM in the Shenzhen metro line 8 tunnel was employed and validated the reliability of the improved index. Therefore, these improved indexes are recommended to describe the boreability characteristics of medium- to strong-quality rocks.</p>