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For any practical wireless multiple-input multiple-output (MIMO) communication systems, the channel estimation can never be perfect, i.e., it is always imperfect. Unfortunately, the channel estimation errors have not been taken into account by existing soft-output two-stage parallel minimum mean square error (MMSE) detector when calculating the log-likelihood of each coded bit, i.e., the soft information. As a result, the system performance can be drastically degraded. In this paper, we propose a novel soft-output two-stage MMSE MIMO detector based on random vector theorem, which takes the channel estimation error into account in the computation of the MMSE filter and LLR of each coded bit. When compared with existing two-stage MMSE MIMO detector, our simulation results show that the proposed novel detector can remarkably reduce the residual error floor and obtain significant performance gain at the cost of negligible increase of complexity. Furthermore, it is observed from our simulation results that the proposed detector is not sensitive to the inaccuracy of variance of the channel estimation error. Therefore, it can be a promising candidate to be applied in practical systems.