The mechanical properties of cold-formed steel at elevated temperatures are the major parameters for both fire protection design and numerical analysis of cold-formed steel structures. Previous research has been mainly focused on the steady state test method. However, transient state test method is more realistic to actual fire conditions. This paper presents the details of a transient state experimental study on G550 steel with thickness of 1 mm at elevated temperatures. The results show that： 1~ the reduction factors of ultimate strength for G550 steel obtained using the transient state method are close to those using the steady state method at temperatures blow 525~C, however, the former becomes much higher than the latter at temperatures above 550~C ; @） strains from the transient state method for G550 steel are higher than those from steady state method under the same temperatures and stresses, especially for the inelastic portions of stress-strain curves. Hence, transient state method is not equivalent to steady state method for G550 steel. The elastic modulus and yield strength of G550 steel from the transient state method are also compared with those from the steady state method. In addition expressions for calculation of both reduction factors and stress-strain curves at elevated temperatures are put forward by numerical fitting, and they are in good agreement with the test results.%冷成型钢高温材料特性是进行冷成型钢结构抗火设计及数值模拟的重要参数。现有的钢材高温材性数据大多基于稳态试验方法得到，而瞬态试验方法显然较前者更接近实际火灾情况。本文利用MTS810试验系统对1mmG550冷成型钢进行了详细高温瞬态试验研究，并将试验结果与已完成的同批次G550冷成型钢高温稳态试验进行了细致对比，结果表明：①G550冷成型钢瞬态试验极限强度折减系数在525℃以下时与稳态试验结果比较接近，但在5500c以上时明显高于稳态试验结果；②相同温度、应力水平下，G550冷成型钢瞬态试验应变值高于稳态试验结果，尤其在应力．应变曲线非弹性段二者差异明显。因此，瞬态试验方法与稳态试验方法对G550冷成型钢并不等效。然后，基于统计平均方法得到瞬态试验高温弹性模量以及屈服强度，同样与稳态试验结果进行了比较；最后，通过数值拟合提出了G550冷成钢高温材性折减系数及本构关系表达式，表达式与试验结果基本吻合，满足工程精度要求。
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