The purpose of this paper is to develop an algorithm achieving the performance of fixed-complexity decoder (FSD) with much lower complexity than FSD. An adaptive expansion strategy (AES) is proposed for MMSE Ordered Successive Interference Canceling (MMSE-OSIC). The expansion degree varies according to the channel condition, especially, partial expansion (PE) is proposed in this paper as a transition between Single expansion (SE) and full expansion (FE). Widely linear processing, which is usually confined to non circular signals, is extended to circular signals in PE. Simulation shows that the proposed AES can significantly reduce the complexity with little performance loss, compared with FSD.