Aiming to further improve the versatility, mobility, and field performance of the phenotypic information acquisition robot chassis, a mobile variable-track chassis was designed based on the actual needs of field crop phenotypic information acquisition in China. Firstly, the overall structure, working principle, and technical parameters of the chassis were established according to the agronomic requirements for field crop planting and driving terrain conditions. Next, the design of key components, such as the chassis drive system, power system, and shock absorption suspension, was carried out, along with component selection and parameter verification. A chassis variable-track walking control system was developed, determining the control logic for track adjustment and walking transformation. Finally, a prototype was built to conduct performance tests. The results showed that the chassis exhibited excellent straight-line driving performance, with an average deviation rate of less than 0.60% on hard surfaces and less than 1.26% on field surfaces. It demonstrated good steering maneuverability, with a single turn offset less than 3.52 mm for in-place turning on hard surfaces, an Ackermann steering turning radius less than 1.76 m, and a single turn offset less than 5.18 mm for in-place turning on field surfaces, along with an Ackermann steering turning radius less than 1.77 m. The variable-track width accuracy was also commendable, with maximum track error less than 0.01 m, making it adaptable to crops with different row spacings. The chassis showed good passability, capable of overcoming 120 mm vertical obstacles, thus meeting the requirements for navigating complex terrain like field ridges and headlands. Overall, the machine’s performance met the requirements for field terrain management operations, providing a comprehensive application platform and technical support for effective field management in wheat, corn, and open field vegetable cultivation.