The invention relates to a string trimming method and a string trimming device. The method comprises the following steps of: replacing each of the operation relationship symbols in the first chargingexpression with a character corresponding to the same, and acquiring the converted second charging expression; receiving a trimming instruction, screening and deleting characters corresponding to theoperation relationship symbols logically related to the first accounting charge ID in the second charging expression and the first accounting charge ID according to the preset rule, and acquiring a third charging expression; and inversely transforming the characters corresponding to the operation relationship symbols in the third charging expression according to the preset order, and acquiring a fourth charging expression. Through the above manner, the accounting charge ID to be deleted in the charging expressions may be quickly deleted without affecting the logical relationship of the entireexpression.