Abstract: In a grounded space environment simulation vacuum chamber,experiments are carried out on engineering models（EM） of space solar arrays.We investigate the initial voltages of the primary electrostatic dischargings（ESD） of the samples in different circumstances,such as the vacuum,gap width and the conductivity of the insulators,and so on.The initial ESD voltages of EM space solar arrays are measured with the DC flashover measurement system in the vacuum chamber.Moreover,the conductivities of the materials used in present work are measured with the American Society Test and Material（ASTM） method.A minimum value of the initial voltage of primary ESD will be obtained,when the product of the vacuum pressure and the gap width（p·d） is about 6～8（×10^-3 mm·Pa）.In addition,the larger the conductivity of the insulator,the smaller the gap width of the minimum initial ESD voltage is.