Abstract: The power density of products in communications and electronics industries is increasingly high. As a result, the traditional cooling way of natural convection can not meet the design requirements and forced cooling by fan is very popular. However, due to limits of the product size, flow requirements and noise，fans working in series tend to be adopted in the product. There is a difference between the actual characteristics curve and the calculated characteristics curve in theory of fans in series at present. The effects of the fan type, the rotation speed and the space between fans on the characteristics curve of fan in series are discussed in this paper on the basis of the fan test standard ANSI/AMCA 210—2007. In addition, the application of fans in series to the system is studied. And some suggestions on how to apply the fans in series to the system in order to get the maximum cooling efficiency are put forward.