Crawler chassis is widely used in agricultural machinery due to its low ground contact stress and other advantages. However, agricultural machines usually work in fields and simple roads, so that the crawler chassis with a fixed ground clearance and angles can cause problems such as bottoming when crossing field ridges. In recent years, various adjustment mechanisms have been developed to for the crawler chassis to improve its trafficability, but most of them have overlooked the changes in track length during the adjustment process. Excessive extension of the track will significantly increase stress and the risk of detachment, while insufficient extension can easily lead to track detachment. Therefore, this study proposes an surrogate-based trajectory planning method for adjusting the track chassis of agricultural machinery, and verifies it with a combine harvester as an example. First, construct two surrogate models to approximate the relationship between track length/adjustment angle and hydraulic cylinder length. Second, under the constraint of track length, calculate the target length of the hydraulic cylinder by adjusting the angle according to the target. Third, discretize the length trajectory of the hydraulic cylinder into some potential solutions and optimize them by minimizing the changes in trajectory length during the adjustment process. Finally, based on the optimal discrete solution, cubic spline interpolation is used to generate the final adjustment trajectory of the hydraulic cylinder. The experimental results showed that the planned track length change decreased by 46.98%, with a percentage change of less than 0.15%.