1. An image compensating method, comprising: using a plurality of scanning lines to scan a document and a longitudinal black and white pattern, in order to produce a plurality of actual gray level values for a plurality of pixels with respect to each of the scanning lines and the document, as well as a correctional gray level value for complete black and a correctional gray level value for complete white with respect to the longitudinal black and white pattern; and obtaining a compensational gray level value with respect to the actual gray level value for each of the pixels according to the correctional gray level value for complete black, the correctional gray level value for complete white, a theoretical gray level value for complete black, a theoretical gray level value for complete white, and the actual gray level value for each of the pixels.