Auto-focusing is one of the key issues in automatic microscopy. The traditional methods suffer from the small search range and are sensitive to the starting position. An autofocusing algorithm for guiding automatic search of the optimal focal plane under large search range for microscopy with small depth of field is proposed. The autofocusing procedures are divided into six types, including two basic types and four complex types, based on the correlations of the defocus distance, the search direction, the search range and the parameters of the focusing curve. Then, the complex types are transformed into the basic types using the changes information of the values of the focusing functions. Therefore, the large search range is transformed into small search range and the traditional extremum search method, i.e. Gaussian curve fitting method can be used for guiding automatic search of optimal focal plane. Experimental investigation has been conducted to demonstrate the validity of the proposed method.