This paper proposes DARSD, a novel unsupervised domain adaptation (UDA) framework for solving the domain shift problem in time series analysis. Unlike existing UDA methods that treat features as individual elements, DARSD addresses the UDA problem from the perspective of representation space decomposition by considering the internal composition of features. DARSD consists of three main components: (I) an adversarially learnable common invariant basis that projects features into domain-invariant subspaces; (II) a circular pseudo-labeling mechanism that dynamically separates target features based on confidence; and (III) a hybrid contrastive learning strategy that simultaneously enhances feature clustering and consistency while mitigating distributional differences. On four benchmark datasets (WISDM, HAR, HHAR, and MFD), DARSD achieves optimal performance in 35 of 53 scenarios, ranking first in all benchmarks, compared to 12 UDA algorithms.