In this paper, a mathematical model of the hydrological cycle of the forest set up on the basis of a man-made forest which originated from a bog land in the suburbs of Edinburgh in southern Scotland is described. The whole process is abstractly recognized as a forest ecosystem with 5 compartments, namely, trees, grasses, ditches, ditch slopes and ditch bottoms. With the aid of some methematical formulas and computer techniques, the hydrological process of the system, which includes flow interception, surface flow, stared in soil, groundwater and the other hydrological phenomena, can be systematically simulated in a quantitative form. The amounts of rainfall and evaporation are put into the model and the runoff water is drained away from it- It is given here for an example to simulate the hydrological process of the system in 1986 and check the simulation efficiency. Through comparing the computed and observed flows of the forest ecosystem, it is found that the simulation efficiency is 93.5% on the daily basis and 85.1% on the two-hour basis.The model reveals the mechanism of the hydrological cycle of the forest. We can estimate the runoff by using the amounts of input pricipitation and evaporation only. It will be helpful to understanding the hydrological effect of the man-made forest which originates from a bog land.