This paper presents a new true triaxial apparatus with mixed boundaries for soft soils. A novel rigid–flexible loading device with four sliding rigid plates is designed. In the new loading device, the sliding block and the loading plate are separated, which allows the plates to be enclosed by a rubber membrane. This unique design can not only avoid the interference between adjacent plates and the corner effect, but also achieve a full π plane stress path. The friction between the sliding plates and specimen is reduced to a negligible level by using Teflon films together with lubricant. Paired sliding plates are connected precisely by a special transmission device and driven by a servo motor, which makes it possible to keep the specimen central, in either displacement control or load control modes. The new apparatus was used to investigate the mechanical properties of Shanghai Layer-4 clay under the three-dimensional loading condition. Typical test results are presented and discussed comprehensively. The influence of Lode angle on the stress–strain relation of the clay in finite deformation is investigated and the applicability of the new apparatus to soft soil testing is confirmed.