This paper examines the validity of the contribution of entropy elasticity, which has been vaguely considered to be the main cause of the thermal expansion behavior of the untwisted fiber. Thereafter, we indicate the inconsistencies and propose a new possible physical principle. A new high-order structural model that can effectively cause anisotropic thermal expansion of fibers is proposed. The observed results verify our proposal, and further show its validity and correctness. The twisted fiber actuator, which has garnered significant attention in recent years, is driven by the anisotropic thermal expansion of an untwisted fiber, which is the precursor of the twisted fiber. Analyzing the physical principle of the thermal expansion behavior of the untwisted fiber is to obtain the knowledge for improving performance of the actuator.