Tetranychus truncatus is an important agricultural pest mite, with a wide range of host plants, strong adaptability, and increasingly severe damage. In order to clarify the adaptability of T. truncatus to different hosts, the population fitness performance of T. truncatus, an adaptive strain of kidney bean, on three non-original hosts—cucumber, corn and jujube—was compared through host-switching experiments. Key biological indexes such as egg stage, larval stage, nymph stage, adult stage, fecundity, and population growth parameters across different treatment groups were quantitatively analyzed. The experimental results showed that T. truncatus completed its entire life cycle on the three host plants after host transformation, but its population fitness exhibited host dependence. Among them, the cucumber host group showed optimal adaptability, characterized by shorter development duration, higher reproductive capacity, and increased intrinsic growth rate. However, the fitness on corn and jujube declined, manifesting as prolonged development duration and reduced reproductive capacity. It was speculated that these differences were related to defense compounds, nutritional composition, and physical structure of different host plants. In summary, T. truncatus had the best fitness on cucumber, followed by corn and jujube after host transfer. This study preliminarily revealed the adaptive differences of T. truncatus on different hosts, which not only provided a theoretical basis for predicting its field population dynamics and cross-host transmission patterns but also offered a scientific support for constructing a regional accurate control system.