A method and an apparatus for calculating blood pressure with signal transformation are provided, wherein said apparatus comprises: a pressure sensor, a pressure and pulse wave amplification circuit, an A / D converter, a microprocessor, a control circuit, an air bump and a valve component connected with the cuff gas circuit, a signal processing module and a ratio analysis module; and the method includes the steps of carrying out signal transformation of a time sequence of a sampled PW digital signal, calculating a maximum amplitude value and a corresponding frequency value based on the transformed signal, taking the maximum amplitude value as a amplitude value under the present pressure, carrying out continuously signal transformation of the time sequences of the PW digital signals at different pressure in order to constitute a PW amplitude-pressure curve and obtain an average pressure and a corresponding pulse rate from the PW amplitude-pressure curve, and finally calculating the systolic pressure and diastolic pressure with ratio method. With the method and apparatus according to the present invention, the measurement error can be reduced in case of the shiver, muscular fibrillation and the like of the human body being examined, and the measurement accuracy of blood pressure can be improved.