This study aims to explore the ultimate bearing capacity of expanded anchors in weak strata, with the goal of addressing the issue of insufficient bearing capacity in traditional anchoring techniques in such strata. Through theoretical analysis and field testing, this paper provides a scientific basis for the design and application of expanded anchors, with the expectation of enhancing the safety and reliability of slope reinforcement and excavation support. Using slip line theory and the effective length principle, formulas for calculating the ultimate bearing capacity of single and multiple expanded anchors were derived and validated through on-site pull-out tests conducted on an embankment slope of a highway in Guangdong. The field test results showed that the ultimate bearing capacity of single-plate anchors was 1.42 times that of ordinary anchors, while that of double-plate anchors was 1.68 times. The comparison between theoretical calculations and field test results was generally consistent, and pre-stress loss tests suggested increasing the tension force of single-plate anchors to 140% of the design tension force before locking, and for double-plate anchors, increasing it to 120% of the design tension force. Expanded anchors significantly improved the bearing capacity of the anchoring system by increasing end-plate and side friction resistance, performing better in weak strata compared to traditional anchors.