The invention discloses a method and device for storing historical trend data in limited memory space. The method comprises the steps of judging, subtracting, comparing, and deleting. In the judging step, judgment is made to the remained space of the memory. In the subtracting step, when the result in the judging step is negative, subtraction is done in a manner that all the data in the memory space minus the adjacent data respectively to get the difference value, wherein the adjacent data is the data which is adjacent when the data storage occurs. In the step of comparing, comparison is made between all the difference values to acquire the smallest difference value. In the step of deleting, one of two mutually adjacent data corresponding to the smallest difference value is deleted, thus leaving space for storage. According to the method, when the memory space is determined to be inadequate, through the deletion of either one of the two mutually adjacent data corresponding to the smallest difference value, memory space can be vacated for storing new data, at the same time the data variation trend in the memory space can be preserved to the maximum.