The variable step size database method for the function synthesis of spherical four bar crank slider mechanism was presented. Based on the analysis of input and output function of the spherical four bar crank slider mechanism, the immanent connection between output function and its corresponding harmonic characteristic parameters was discovered. Based on this discovery, a numerical atlas with variable step size database of output angle function, which contains 42185 groups of basic dimensional types, could be created by the harmonic characteristic components and associated basic dimensional type of spherical four bar crank slider mechanism. The best suitable basic dimensional type of spherical four bar crank slider mechanism for prescribed function was decided by using fuzzy identification method and numerical atlas database. Thus the function synthesis of spherical four bar crank slider mechanism was made possible. In order to improve the precision, the result could be optimized by genetic algorithm. The ventilation device of the greenhouse was taken as an example to illustrate the analysis process and application of this method.