The invention discloses a character conversion rule generating method and device, and aims at solving the problem that a speech recognition system fails to identify case data broadcast by voices into special case characters accurately. The method comprises that an original text is converted into voice data and then converted into a conversion text by speech recognition; the conversion text is compared with the original text character by character to identify a different string; a string in which the different string is included, the length of the string is shorter than a preset string length, and at least one of left and right information entropies of the string is lower than a preset information entropy is extracted from the position where the different string is positioned in the conversion test and serves as a conversion string; an original string in a position corresponding to the position where the conversion string is positioned in the conversion text is obtained from the original text; and a correspondence between the original string and the conversion string is established to obtain a character conversion rule. According to the method, the conversion rule of special strings can be generated according to present case data and can be used by the speed recognition system.