The present invention provides an image based blood pressure monitoring method, comprising: acquiring at least a human image information of at least a human skin area; according to the human image information to locate at least a ROI; extracting the human image information of the ROI, and calculate; filtering the average value of the human image information; monitoring the filtered signal; calculating an image pulse transmit time of the filtered signal, and calculating an inter-beat interval of the filtered signal; and employ the specific prediction model, to calculate a systolic pressure value and a diastolic pressure value.