This paper presents a new method for the synthesis of spatial mechanism. Using harmonic analysis, the coupler curve and output angle function of spatial mechanism can be described as a series of harmonic characteristic component. For the path generation problem of the spatial mechanism, a technique of normalization processing eliminates the effects of scale, translation, and rotation; in other words, the harmonic characteristic component of the coupler curve is just determined by the basic dimensional type of spatial mechanism but not by the real dimension and the installing position. The numerical atlas database of coupler curve or output angle function of spatial mechanism can be created by the harmonic characteristic components and associated basic dimensional type of spatial mechanism. The best suitable basic dimensional type of spatial mechanism for prescribed function can be decided by using fuzzy identification method and numerical atlas database. On the basis of the basic dimensional type and formulas, which compute the coupler point, true size, and installing dimensions of the spatial mechanism, the dimension synthesis of the spatial mechanism is made possible. This paper takes the RSSR mechanism and RRSS mechanism as an example to illustrate the analysis process and application of this method.