Abstract: The farm implement unit of tractor is a complex power system. Under the condition of random environment and load wave, the traditional static analyzing method can not make tractor best match with diesel engine, thus dynamic analyzing method is its necessary trend. The research on the dynamic problems of diesel engine system is one of the major problems which need to be solved on making dynamic analysis of farm implement unit. In this paper, the differential equation of linear model of diesel automatic-control system and the transfer function whose input is dynamic load and output is rotating speed were developed. The frequency characteristic of system is deduced and the calculating examples are given. It is shown that the frequency characteristic of diesel engine system is a function of the working point of itself. When the diesel engine matches with dynamic load, their frequency characteristics should be noticed. It can lay a foundation for the dynamic matching of farm implement unit.