The invention discloses a high-precision online measuring instrument and a measuring method for the strip shape of a plate and strip, and relates to the measurement of the strip shape of a metal plate and strip. The method comprises the following steps of: distributing 3 beams of line laser along the motion direction of the plate and strip; irradiating on the surface, in the width direction, of the plate and strip to form 3 laser irradiation lines, wherein the included angle between the line laser and the horizontal plane is 30 to 60 degrees; shooting the changes of the positions of the laserirradiation lines on the surface of the moving plate and strip at a high speed by using three full high-definition surface scanning digital cameras above a laser irradiation area of the plate and strip; respectively sending the changes of the positions to a digital signal process (DSP) to obtain the position of a laser spot on each measurement strip; calculating to obtain the parameters of the strip shape of the plate and strip by using a measurement control cabinet; displaying the parameters on a screen of a computer; and storing the parameters in the computer. The instrument and the method have wide adaption range, high measurement precision and high measurement speed, can be used for measuring the strip shape of a cold or hot rolled metal plate and strip, and can ensure long-term reliable and stable operation in very bad environment of metallurgical production.