A parabolic trough solar receiver is the key component in the parabolic trough solar system. The annulus between the absorber and glass envelope is evacuated to protect the solar selective coating and decrease the heat loss of entire receiver. However, at any existing operational solar plants, the vacuum condition in annulus may be compromised by glass breakage, seals and bellows broken, hydrogen penetration and getter decomposition. In this paper, different heat transfer mechanisms due to variable residual gas conditions in the annulus are analyzed. In addition, a series of heat loss tests of China's first high temperature vacuum solar receiver are carried out by two methods with special exhaust system and vacuum gauges. The negative effects of residual gas, including environmental-sensitivity enhancement, coating degradation, and system efficiency decreasing, are acquired and analyzed with theoretical as well as test results. (C) 2011 Elsevier Ltd. All rights reserved.