Abstract: As a type of anti-floating anchor， under-reamed ground anti-floating anchors with capsule features factory-manufactured key components and on-site rapidly assembly technology， enabling efficient construction and reliable quality control. They have been widely used in anti-floating anchoring engineering and are favored by many engineers and technicians because of their remarkable features， such as large anchoring force， small deformation， and multiple anti-corrosion functions. In order to standardize their design， construction， and quality inspection， the Technical Specification for Under-reamed Ground Anchors with Capsule（T/CECS 1259-2023） was promulgated and implemented on June 1， 2023. Addressing key concerns in engineering design， such as determining the ultimate pullout capacity and load-displacement characteristics of under-reamed ground anti-floating anchors with capsule， this paper analyzed the field test results. Based on existing methods for calculating the characteristic ultimate pullout capacity of the anchor plates in sand and clay， this study proposed respective prediction methods and calculation formulas for the ultimate pullout capacity of under-reamed ground anti-floating anchors with capsule. The feasibility of their application without prestress was discussed， offering design guidance for using them as structural tension members in anti-floating engineering.