The optimal design of a symmetrical two-stage flat reflected concentrator (STFC) was investigated. It is composed of two symmetrical off-axis concentrators and inclined flat reflectors. The variable range and its influence on the energy transmission of STFC were detailedly analyzed. The best values were derived by considering a space solar power station (SSPS) without any convection cooling device. When the optical system is moved following the sun, we adopt the approach of rotating the secondary reflectors to cope with change. Afterwards we simulated the concentrating characteristics of STFC by Monte-Carlo ray tracing method (MCRTM). When the two sides' focal spots just coincide, the concentrated flux distribution presents uniform in the extreme. Finally the construction of primary concentrators and effect are discussed for reference. (C) 2013 Elsevier Ltd. All rights reserved.