Spatial modulation (SM) combined with spatial multiplexing is a newly developed transmission technique in multiple-input multiple-output (MIMO) systems. The resultant system, referred to as SM-MIMO, can use the maximum-likelihood (ML) detector jointly detecting the antenna index and symbol vector. As known, the ML detector can achieve optimum performance; however, its computational complexity may become prohibitively high when the dimension of MIMO system or symbol constellation is large. In this paper, we propose a new method to solve the problem. The main idea is to conduct a two-stage detection, antenna index and symbol. We develop a Gaussian approximation method such that the antenna-index can be first reliably detected. An enhanced method is also proposed so that the near-optimum performance can be obtained. Simulations show that our methods outperform existing methods while the detection complexity remains low.