Wheel-type rice transplanters have been widely used in mechanized paddy-field transplanting because of their flexible operation and convenient field transfer. However, during paddy-field operation, the unstable adhesion state between the driving wheels and saturated paddy soil may induce lateral deviation, longitudinal slip, and row-spacing errors. To clarify the slip behavior of a wheel-type rice transplanter under different soil moisture contents, traveling speeds, and whole-machine masses, this study analyzed the variation characteristics of the slip ratios of a wheel-type rice transplanter. A Xinyang 2ZG-6D1(G4) wheel-type rice transplanter was selected as the research object. A whole-machine multi-body dynamics model was established in RecurDyn, and a layered paddy-soil particle bed was constructed in EDEM. The Hertz-Mindlin with JKR contact model was adopted as the soil contact model to describe the adhesive contact behavior among paddy-soil particles and between soil particles and soil-engaging components. Compression tests and direct shear tests were conducted to calibrate the discrete element parameters of paddy soil, thereby improving the reliability of the soil model. The bidirectional coupling between the transplanter model and the soil model was realized through the EDEM-RecurDyn coupling interface. Soil moisture content, traveling speed, and whole-machine mass were selected as experimental factors, while lateral slip ratio and longitudinal slip ratio were used as evaluation indexes. Based on the calibrated discrete element method and multi-body dynamics (DEM-MBD) coupling model, a Box-Behnken response surface experiment was conducted to investigate the effects and interaction mechanisms of the three factors on the two slip indexes. Regression models of the lateral and longitudinal slip ratios were established, and multi-objective optimization was carried out to obtain a suitable parameter combination. Field tests were finally performed to verify the prediction accuracy of the coupled simulation model. The results showed that soil moisture content had the most significant effect on both lateral and longitudinal slip ratios, with contribution rates of 20.54% and 35.74%, respectively. The interaction between soil moisture content and whole-machine mass also had an obvious influence on the two slip ratios, indicating that the effect of machine load on wheel-soil interaction depended strongly on the moisture state of paddy soil. With increasing soil moisture content, the bearing, shear, and adhesion characteristics of the soil changed, further affecting wheel sinkage, soil adhesion, and driving stability. Within the experimental range, when the soil moisture content was 32%, the traveling speed was 0.54 m/s, and the whole-machine mass was 854 kg, the lateral and longitudinal slip ratios showed relatively good comprehensive performance; that is, both indexes remained at relatively low levels under the multi-objective optimization constraints. The field validation results showed that the measured lateral slip ratio was 1.55%, while the simulated value was 1.36%, with a relative error of 12.25%. The measured longitudinal slip ratio was 11.48%, while the simulated value was 10.15%, with a relative error of 11.59%. Both errors were within 15%, indicating that the established DEM-MBD coupling model can reasonably predict the slip ratios of a wheel-type rice transplanter under paddy-field conditions. This study provides a feasible simulation method for analyzing wheel-soil interaction in paddy fields and offers a reference for the design of walking systems and the optimization of operating parameters for paddy-field machinery.